If it's not letting go at the bottom, it's one of two settings. Either the return motion is off, or the thread passage is too tight. Thread passage is where the hook and driver "kiss" at the top. There should only be 1 or 2 thread width between the two. Bending forks are the best way to adjust that. Return motion is like you said, the hook doesn't quite rotate far enough before it starts back the other way. Not having one in front of me, I wouldn't be any help with that. I hope someone with more Singer knowledge can help.