You will have to remove the passengers side interior door panel and check the locking mechanisim.
You will have to remove interior door handle, door latch and panel.
Most likely you have a locking rod that is loose or binding.
If you have a local " pick and pull" go there and find a car like yours, remove interior panel and check the lock mechanisim.
This will give you access to parts you need and practice at removing the panel.