Google Indexed URLs Going Down?

Why is a website not appearing in Bing SERPs despite Bing Webmaster Tools telling me it's been indexed?

• The site is a redesign with new URLs. URLs from the old site have all been 301 redirected to the new URLs and all is well within Google. In Bing, however, the sitemap remains pending, only around a quarter of the pages have been indexed, just two search terms  appear in Bing and for old URLs after four months. None of the new URLs appear t all. Your thoughts and help appreciated!

• Answer:

  It's hard to speak in specifics for a question like this, because only Bing's engineers know the exact answer. However, I know that Google and Bing have a "supplemental" index of sites that - while technically indexed - do not appear in the main index and therefore are highly unlikely to appear in a regular search. My suggestion is to work on your backlink profile, creating quality content, etc. and wait for Bing to come around. I also have found that Bing is much slower than Google when it comes to making changes. I've seen cases where Bing took a few weeks to make an adjustment that Google seemed to make in a few days.