Ok,
The fob sends one of two coded radio signals to the car, to unlock or lock.
It sounds like only one of two things wrong.
1. The fob is faulty, so only the lock signal is sent.
2. The immobiliser has a faulty output to the door locking solenoids, maybe a wire off etc, so it can only operate it in one direction.
Does it unlock all doors off the car key? If not the solenoid or door switch may be the problem but if it does, its fob or immob.