Locking issue

The driver side door on my 1993 chevy lumina 2dr. is having some issues. the lock on the door is screwing up. if the door is shut and is locked ,it wont open when unlocked from inside or out. After fighting it for a few hours to get it open, it still does the same thing. the latch doesnt open when locked then unlockled,, if i leave it unlocked it does fine. but out of habit i lock it and i know its gonna happen more, any ideas of what is going wrong?