Sometimes, and I don't know why because the guy who serviced my machine didn't tell me, but the tension can slip. My old Singer was notorous for it, and finally the tension disc just couldn't do what it needed to do and I had to retire the machine. Maybe the problem is in the tension contraption and not YOU. We all have times where it seems like it is going great and then wham! Don't be down on yourself. It could be the machine. It could also be that you might be going too fast or moved the quilt too fast.