n/s/r door unlocks but wont lock !!!!!!

eyup ducks,my passenger side rear door wont lock with the central locking but it will unlock,most annoying,im having to drive round with the interior panel removed,ive tried another secondhand solenoid to no avail,if i help the rod when locking it will lock,could it be a knackered lock or does it just need a good wd40'ing...............

Wunt hurt to give it a good wd40ing.

could be the spring?
hopefully someone else can clarify...

look here if it is: Landrover Discovery door lock repair spring 3 door set on eBay, also, Land Rover, Car Parts, Cars, Parts Vehicles (end time 24-Jan-08 22:00:24 GMT)