The system of door openings is controlled by a module known as the central locking module, this is responsible for controlling doors, hood, rear-view windows (according to the model) etc. There are certain aspects that may be happening in your vehicle:
Starting with problems of battery not only of the key fob but also of the vehicle, when it has problems in the power supply of the battery the modules do not receive the voltage required for its operation, another important aspect is that the key fob battery no longer has charge, this happens a lot. These batteries have a short period of time, so they must be replaced frequently to work properly, you should consider the key fob, although as it is only in just one door is it unlike to be the reason of the problem.
Another cause can be the wiring or the soloneid of the locking of the door, this tend to fail someties due to loose connector, split wiring or that the solenoid is not working, this has to be checked with a tester to check the signal to see if it is arriving properly.
My recommendation is to start with a computerized diagnosis first, because this can rule out several problems in the electrical system and modules, you can also reprogram the key fob which is usual to get deprogrammed. The diagnosis will also show if the problem is with the module itself or with its wires or connectors that sometimes produce the problem you have.
This works can be done for locksmith and specialist in this system, as well as the dealership.
If you do not want to scan the vehicle start as i told you with the check of the solenoid its connectors and its wiring.