I have no idea I'm afraid but have you tried unlocking the doors with the key rather than pressing the remote control buttons?
There is a feature (at least on mine) that seems to be intended to guard against accidentally unlocking the car. If you unlock the doors and then don't open any door within a set time (I think it's a few minutes) it locks itself again. Perhaps that timer has gone wrong? (Unfortunately I have no idea where it is or how it works/how to troubleshoot/replace it)