What could be causing a 44% organic traffic loss after a migration from PrestaShop to Magento?

You mentioned redirects, which I'm going to assume to mean that the structure of your site was changed along with the file names for the pages (i.e. urls). This alone with kill a site's organic traffic for awhile until Google and the other engines reindex the site entirely. Using redirects is the proper way to handle it, but there is still always a "disturbance in the force" for at least a month, but maybe longer, as the bots consider these brand new pages with little equity. Also, noticed you have a fairly complex robots.txt file, which is not bad in and of itself, but could accidentally be blocking something you don't want blocked. Check google webmaster tools and see what is being blocked from crawling to see if it's something that you did not intend to block. Otherwise, lots of great work on the optimization. Good luck. Be awesome.

When you did not retain your original URLs, you disappeared from the indexes of all the search engines.  Your home page remained the same because it was your domain name.  However, even that could change if you went from /index.php to /index.html or some other extension.  With all new URLs, any back links you had disappeared as well because they became "dead links". In effect, it was like you had not existed prior to the change to the new platform.  The new platform may also have changed some of the structure of your website.  For example, how a product that appears in more than one category is handled.  Or, how you set-up your categories (adding more, removing some, etc. with the new platform options). What likely remained the same were things like "About Us" or "Contact Us" pages or maybe a random category or product page or two that were flukes. When changing platforms, you have to deeply evaluate the impact on  your URLs with the big question being:  can you keep them "as is".  Many platforms have a feature or a module or plugin that will allow you to keep your original URLs.  Sometimes you can work with a programmer to create this option.  However, some platforms are simply not set-up to allow this possibility.  You have to decide if the features of the new platform are worth the loss in your search engine results and the projected losses in traffic.  This is a much bigger question than most people realize.  New platforms can look very "sexy" with their features, but the impact on your traffic can seriously impact your revenues up to and including resulting in you going out of business from the drop in revenue. If you must move and you cannot retain the original URLs, using a 301 redirect for EVERY page on your website is the way to go.  However, that's a big task and people who don't understand the significance will try to talk you into simply doing a 301 redirect that sends all traffic to the home page or to a category page.  Bad idea.  Each page should go to its equivalent on the new site. The best way to change platforms is to create as close to an exact duplicate as you can of your site on the new platform and keep your original URLs or do a 301 redirect for every page.  Do not try to implement any of the new features or make ANY adjustments to how your products or categories are set-up (yet).  The first goal is to move the site without losing your traffic or your ranking in the search engines.  Once you make the move and evaluate its success, giving it time so you can really see the impact with your search results, THEN you can start to take advantage of the new features, one by one, so you will know the impact.  If you try to do it all at once, that will make it difficult to figure out what's causing any problems and to be able to take corrective action. This is one of the reasons it's so important to spend the time upfront in selecting the right platform so you can minimize the need to move.  Of course, when you're starting out, you don't know or fully understand what you'll need and it's easy to get swayed by price or some other factor that seems important when you don't know enough to realize that it's not.  Even in a "do it yourself" situation, it's worth consulting with a professional to get educated so you can understand what factors should be considered during  your selection process.  There are a lot of articles out there touting various platforms and also doing comparisons, but finding an up-to-date article that really digs deep and covers the full realm of what an online retailer needs in a store (beyond the obvious basics) can be a challenge.  Add to that the factors related to your specific business, like time available to keep the site updated, knowledge of coding and/or the interest (or lack of interest) in learning some of the basics, etc. can all make a difference in your choice of platform.  And frankly, sometimes you have to step up and learn new things if you are serious about having a highly profitable store. But, I'm "preaching to the choir" (smile) because you already know this; so it's for the readers who may be considering a move or even just getting started. All the other things you mentioned in your post are noteworthy and can be helpful in regaining some of what you lost, but the core issue is that you did not retain your original URLs and thus disappeared from the search engines, in effect, making your store "start from scratch". Unfortunately, with six months having gone by, your original URLs are now gone from the indexes as well, so a 301 redirect at this point is only helpful for those back links on sites that weren't keeping themselves up-to-date enough to know they had a dead link on their site.  I'm guessing they weren't sending much traffic anyway.

You lose that much traffic the day after a new site launch and you should put the old one back until you work out the details of "why". That would be water under the bridge now it's 6 months later. Echoing the guys above, sounds like URL changes with no redirects... but:  Did your 404 errors spike? Did your server errors spike? Did your index page count drop? Did you noindex the development site & not remove soon enough i.e. at launch? The latter I've seen too many times during a new site launch with similar results... even removing after giving Google the directive for a few weeks can take a massive amount of time. Did your new navigation add additional 500+ links on each page which Google essentially couldn't / didn't crawl based on server issues? It's a lot of top nav. You can leverage tools like SearchMetrics or SEMRush to see historical data and ascertain where the primary drops were in the SERP... sounds like you have the data to do that yourself too... I'd focus in on what dropped, if it's across the board I'd have to lean toward user error and say the way back is to keep plugging.. you lost Google trust, diminished your authority and it's a long road to rebuild. HTH

I have read your complete post. I fully understand that your website drop traffic 44 percent. I don't know what is the actual reason but I think this is hummingbird update penalize your website keywords rankings. There are some reasons of search engine penalty. 1- Website unnatural content: Please recheck your website content optimization. Mostly hummingbird update penalize bad content pages, copy paste content, unnatural pattern content, over keyword density, and irrelevant content pages. 2- Keyword stuffing: Unnatural keyword stuffing also decrease your website keyword indexing. Please recheck your website keyword relevancy and remove unnatural and huge meta keywords. Also please add fresh and relevant meta text description 3- Bad linkbacks: I understand that you will not apply a bad backlinks method on your website but please view your website off-page SEO strategy. Google's allow only natural pattern link building method. Only relevant and high page rank websites links are good for website health. Fully analyse your website backlinks and if you find out bad reviews, lower then page rank 3 links, and bad category pages backlinks so remove it immediately. Google hummingbird update analysis method is simple. Compare your website previous google index pages to recent index pages and analysis your website previous keyword rankings to recent keyword rankings.