Well probably due to moister and rust etc the door locking latches are stuck or a metal arm leakage for the door handle has come off or bent etc.I'd remove the screws and pop the door panel off, weather stripping and place a light close by then try the different door handles and latches to see what's going on but more then likely you need to spray the whole locking system and soak it really well , while you keep trying each door handle inside and out of car and all switches etc too.
I'd do this to every door on that vehicle and the back door that is stuck closed I'd remove the screws and give the door panel a good tug to remove it , believe me they can come off even with doors closed bec I've done it before myself , I had both back doors stuck like gold on one of my cars before.
Hope that helps and best of luck.By the way buy the real good spray from a cars parts store spend the 10 to 12 bucks on that stuff bec it works really well.Keep in mind that while you spray these door lock assemblies etc be sure to move the handles etc rapidly at same time to allow the spray to get to the levers on the lock assembly fully , plus you really should allow the spray to sit for several hrs, then spray and move the latches etc again.
As a preventive maintenance I'd spray the locking system a couple times a year and you can get spray in them without removing the door panels but the best way is by removing them.
