Are your feed dogs up? Sometimes when I'm cleaning my bobbin area, I lower them and forget to raise them again.

I was going thru fits trying to solve a problem with my top stitches. Everything I did would not work. Tried rethreading, using new thread, reloading bobbin, different foot, every trick in the book. The I remembered the old saying of problem on top, check the bottom. Took it all apart and did another cleaning (already done that!). But, this time I also used a strong light and a mirror. Guess what? I found a tiny piece of thread caught underneath. It was causing all the problems!!!! Now, whenever I clean, I finish with bright light and the mirror!

I've learned the same thing. I was having problems with the 301 which is one of my most problem free machines and got my trusty little LED flashlight out. There was a tiny piece of thread stuck in the bobbin area. I pulled it out, oiled, but the bobbin back in and life was good. Ditto with my Bernina 180 and I even had that experience with my new Juki. It started stitching badly after having the top thread break. I finally got the light out and looked. Sure enough there was tiny piece of thread stuck in there. As soon as I removed it, all was well. It amazes me how a tiny piece of thread in the wrong place can cause so many problems.