Problem with fuel tank door-1998 323is

I have a 1998 323is.

All doors (including the Front and the trunk) are functioning perfectly- lock and unlock but not the Fuel tank door.

The fuel tank door can be unlocked but CANNOT be locked. I though it was something wrong with the lock actuator and replaced with a new one. But it did not work - it can be unlocked but still not be able to lock.

Can anyone kindly advise how can I fix this problem?
thanks

it does not lock when you use the key in the drivers door to lock the car?

it should lock when you lock the door