That happened to me the other night. I started checking diff things to see what it could be, changed all of it, & still did it. I took the bobbin out a final time & low & behold I had Emb thread I'd used last for an Emb Monogram (in bobbin) & Coats Clark in top thread. Once I changed the bobbin thread to CC it worked fine.
Don't you want to question your sanity when these things happen? I had read about the threads in an Emb mag, so thank goodness I had or I'd have thrown the project in the corner. You sound like most of us, hate to take out all the bobbin loops & stitches. Sometimes I've had my 2.5 stitches change into very tiny, tiny stitches & still don't know what causes that. I think I keep forgetting to change my fabric type - woven light, med, heavy, stretch, leather, vinyl. I don't often sew stretch, but occasionally a customer will bring in a piece & I find it difficult to work with, unless it's a light weight. Hated back in 60s, 70s, 80s when the thicker polyester was popular. It didn't wrinkle, but it sure was the devil to sew the seams (for me that is).