Mistakes & fixes

Thread fraying into fuzz or snapping into short pieces usually means it's catching on a burred needle eye or thread guide, or is old and brittle. Replace the needle, check guides for nicks, use fresh thread.

A raised ridge forming along a line of stitching on stretch fabric happens when tension is too tight or the stitch is too dense for the fabric's give. Loosen tension and use a walking foot with a stabilizer.