I have seen 3 new norinco rifles that came from different retailers (whom all sourced their rifles from the same distributor) that had out of spec bolts that drop below the safety bridge when fully run to the rear of the action. All rifles show the same symptoms, they all do it occasionally with the trigger group installed, but do it every time with-out fail when the trigger group is removed (ie when doing the tilt test).
Not that I think this is happening with your rifle OP, but be aware that this maybe an issue.
1st ensure you have her lubed properly, make sure that it's not filthy. No oil in the gas system as already mentioned.
Still doing it? Check the magazine. Try a different mag that works in a different rifle, if that solves the problem, then that's your answer.
If it's still doing funky chit...... Then now it's time to start going through the stickies and doing all the checks and tests.....
If that still doesn't solve your problem..... You may need the services of a professional!