Originally Posted by
Blackberry
there are two problems, it randomly skips stitches and the top thread doesn't readily release when the material is pulled from the machine at the end of a seam.
Some Singer 15s (earlier ones) thread left to right, later ones with reverse thread right to left. Your clone has reverse, so how are you threading it? Do you have the manual? Needle in the wrong way and threading the wrong way are the most common reason for skipped stitches.
The generic manual I have suggests flat to the right, thread left to right, unlike a Singer but I would try both ways. Whichever way the needle is in, the thread is inserted from the other side.
Top thread not releasing is usually caused by rust or dirt in the tension discs but sometimes it's just that the release pin is worn down.