Sounds like you're taking about eyelashing? Oh the horror. I found it was usually the presser foot not being down or the thread coming out of the tension discs. It can slip out but appear to be fine. This is why it's important to know your machine, what it sounds like when it's doing a proper job, vs what it sounds like when it's not. This way you know when to stop and check. The first quilt I practiced on had horrible eye lashing but was fine on the front. So I covered it up and called it a win. NEXT