That happens to me often. And I HATE it! I don't get why the top looks perfect, the machine makes no odd sounds and sew perfectly, yet the bottom is all messed up! I dont' get it! But...the solution is...completely re-thread the TOP thread. Stupid it sounds and I don't see why, but that works every time. I don't understand it, but as long as it works! I now even will "thread head" a bit longer to make sure the bottom thread is doing right before I sew an entire seam (or even worse FMQ an area!) only to find out half my spool of thread is on the bottom! Thing is... the top thread stitches are secure and you'd think it'd be easy to tear it out but it isn't. But just get your seam ripper and while watching a TV show one evening, pull it out. Not hard, just time comsuming and messy with all the threads but worth it. You won't notice it once out and can re do it just fine.