Hello Katie, i'm no expert and have never had one of these latches apart, you may have tried a few of these things already, but if not:
I assume you can open the door from the outside with no problem, if so then you need to check that the internal rod/bar/ whatever it's called from the internal latch release handle to the latch turns the cam to the same degree. I just checked mine and it does
If it doesn't, then can you manually pull the bar, to operate the cam to the same degree?
Are the small rubber guide posts all fitted and secure, i would have thought that even if one of these was broken or missing, the bar could deflect enough to not operate the cam far enough, looks like even a 1mm deflection could affect the cam. The rubber guide posts and the small clip that secures the bar to the latch are available from the usual suppliers
Is there any adjustment on the bar, to effectively decrease the length of the bar, and increase the pull of the cam, perhaps at the spring end of the bar?
Does the strength of the spring on the bar have an effect on the pull, perhaps it's grown weak over the years, i know i have
Is the chrome receiver in the door jam fixed tightly, the chrome receiver is rivetted to the back plate and can become loose, i would have thought this would cause a locking problem, but i guess it could cause the opposite effect? My drivers door receiver is very tight, almost no movement, but my passenger door receiver is quite loose, moves a good 2mm in all directions, although it doesn't cause any problems at the moment
Is the receiver aligned correctly, perhaps it needs a small adjustment?
i assume you've given the latch release mechanism a good dosing of cleaner and degreaser?
Just looking at some pictures of the bar/rod, i'm not clear whether the rod pushes the cam to open, so moves from handle to latch, if so it needs to be lengthened somehow to get the extra little push necessary, or pulls the cam to open, so will need to be shortened somehow
