Get some good rust remover and lubricating spray , remove each of the door panels , gain access to the locking system and spray them soak them good , let it settle several hrs , preferably over night and keep trying the locks manually and spraying the whole system and more then likely that will fix this problem.
Hope that helps and best of luck.By the way there is a spray now out that the jet engine mechanics use to free up rusted etc bolts that you can buy at the auto parts stores.