By "overlock" thread -- do you mean serger thread? It's not recommended for sewing machines, has more lint than regular thread and can leave linty residue -- so sounds like it might be a thread problem. Do you sew slow or fast? Might try to change the tempo -- when I first began found that faster worked better for me, now can control going slower and getting enough stitches close together not to get those "eyelashes" on the back when moving the fabric too fast. Good luck - it's a trick to master, but great fun once you get it.