Cari, those usually have some kind of lock down option. If oil gets where the lock down ends inside some times it can slip. Wipe the internal place where it supposedly holds with a little naphtha or some alcohol and see if it holds better. Stitch length can do the same on some machines if you have oil where you shouldn't. It could need a bit of tightening inside. Flashlight/detective work. Oh so agrivating. I think I know of a clone that does tricks like that and I need to mess with it. Maybe need a new thread on fixing that.