When the slide is pulled back from slide lock, it should travel enough farther back that the forward part of the notch in the slide overrides the slide stop and forces it down. It's not in any way spring loaded, so it won't drop of its own accord. The usual culprit when you can't slingshot is a shock buffer, which reduces the rearward travel by the thickness of the buffer.
An exception is Les Baer Commander-size pistols. He makes them so they can't be slingshotted. The reason is that he uses the same frame as his Government size pistols, and won't do the extra 1/10-inch of machining needed to make a Commander work like a Commander.
There should not be a problem with a Norinco. Are you running a shock buffer?