Door Latch Trouble

The back door on the driver's side is having a bit of trouble. It opens and closes fine from the outside, but from the inside it only opens... once. Unless I reach inside the door behind the panel and manually reset the mechanism, the door will not shut properly. Is there something in the mechanism I might lubricate to make it work again, or is there a spring or component missing and I need to replace the latching mechanism?
Any advise will be very much appreciated.

Start with a good spray of WD-40 or similar. That might free off any jammed lever. If not pull the lock and check it carefully. You should be able to spot any problem. Lub all the other door locks too. If one needs it they all do.