Loosening my bobbin tension has no effect. Where am I going wrong?

Thanks, @monopas! I'm just doing two layers of silk. I didn't know Scanfil was considered 'heavy' thread, or that I might need to readjust tension more often because of it. Good to know. Peeps. Peeps. I just learned how to remove the bobbin case entirely on this machine (YAY!!), and cleaned under it with a paintbrush, removed as much lint and gook as I could find. I readjusted the bobbin tension to that point at which there is the slightest resistance if the thread is tugged. I cleaned the top tension discs. I went back to the manual and oiled ALL THE THINGS, then carefully rethreaded top and bottom, with @evilmomlady's tug on the top thread to seat it properly. Results? Like, OMG. NO resistance now anywhere. This thing sews like a hot knife through butter. I still need to adjust the tension on bottom a teeny bit, but my adjustments on top actually affect the bottom thread again in a way that makes logical sense. THANK YOU ALL SO MUCH *gives you all medals and resumes sewing with a giddy grin*

Adjusting bobbin tension falls under "almost never". Top tension, all the time. Also are you using the same thread on top and bottom? Trying to use 2 different threads can make it f up.

Having looked at some information about your thread, Scanfil is heavier than what I expected you might be using on silk, even for cushions. A 30 weight thread needs a 14/90 size needle (big holes!) and would be difficult to run on the bobbin of a horizontal bobbin machine, because the tension of the bobbin case would be set for finer (usually 50 wt or all-purpose) thread. Usually a 30 wt would be used for decorative or topstitching, with a finer thread in the bottom. Anyway, you might want to switch to scraps of something else in order to get the tensions balanced if you want to continue with this thread, because it might take awhile. And check the upper tension disks, maybe run some dental floss through them with your hands to make sure that a bit of stray thread isn't stuck there. It is quite possible that your bottom tension loosened itself after a short time with the unusually heavy thread, which made the top tension "wrong" in response. The weight of thread is surprisingly important here, I think. Going through 4 to 6 layers of tricky fabric with that heavy of a thread, I'm surprised you got 16 feet done! Maybe consider changing to a different thread.

My first thought was to rethread the machine but sounds like you tried that...hmm if your bobbin is lighter than the one your machine came with, that might be causing the issue - if it's not sitting correctly (because it's too light) I think that could cause those loops. Also: -Have you replaced the needle recently? Apparently they are supposed to be changed in between each project, and 16ft of sewing sounds like a lot. - I would also try re-loading the thread on the bobbin.

Sounds like a threading problem. Put the tension back where it was. Then take all the thread out and slowly rethread both the top and the bobbin. Make sure the bobbin isn't backwards and that the top thread is going over that thing that goes up and down at the top.

Sounds like an issue with your top tension rather than your bobbin, TBH, esp. given that it cropped up when you replaced your thread. When you pull each thread from the machine, is the bobbin thread super-hard to pull and the top thread normal, or is the bobbin thread normal and the top thread super-loose? I am guessing the second. I would replace the needle and return the bobbin tension setting to where it was before. Re-thread everything, slowly, with the needle in the all-the-way-up position and with your presser foot up. Try again. If that doesn't work, try (carefully!) dis-assembling what you can and removing any lint from the bobbin area. And then re-thread, again, with the needle all the way up and with your presser foot up. And if that doesn't work, rewind the bobbin or look for the right bobbin(s) for the machine.

if the loose loops are on the underside of the fabric, it sounds like the tension on your top thread is too loose - have you adjusted that?

I've had similar problems due to using wrong thread, or yes, having the feed dogs disengaged without realizing it (without thinking, I was trying to compensate by just pulling the fabric along - are you pulling? You shouldn't need to.).

(Usually it's like 1 drop of oil in the little hole under the center of the bobbin case)