Door Locking problems

[Kane]

Hi, I have a problem with locking the driver front door, central locking motor (5 Door)When using the centre console switch all doors lock except this one, also with the remote control. I can lock the door manually pushing it down, then all unlock ok.
If I lock the car externally with the key then everything locks ok, usually, but now and again the front passenger door lock fails to operate and requires manual locking.
But when locking internaly the passenger door fails to lock 90% of the time, everything else works fine.
Does this sound like the Locking motor is on the way out? any methods to free it up? or just get a replacement.
Thanks in advance, any help would be useful.