225 Passenger door lock not functioning.

MY2005 5-door 225.

Every time the door lock should operate, the servo motor makes a normal sound just like other doors , but the door doesn't lock. On rare occasion it still does though. The same with opening the door, the lock doesn't normally open, but sometimes does. Only way to make sure the passenger door does lock up, is to keep it open and press the locking button on the driver's door, then shut the passenger door and it stays locked until opened from inside.

Complicated, right? Hopefully someone has had something similar to tackle? Any hints what might be causing the problem? Not sure what the normal spec on a 225 is, but this one has keyless everything.