doesn't seem to have the power to open/close. Any help pleeeease?

 

Since I don't know your model year details I assume your vehicle has pneumatic locks.
If the other doors work OK then the PDE (Pneumatic Control Unit) must be OK. This provides the pressure.
Likely therefore that the door lock bellows is perished (or similar) allowing the pressure to escape rather than operating the lock.
Alternatively the pressure could be being lost along the tubing from the PSE unit.
Hope this helps.
Stuart