Sounds like you have a bashed up latch mechanism. They get damaged over time as the hinge pins wear, then the door sags, then the rotor mechanism slams into the striker plate. You might just have a bad or broken spring.
I have reworked some pretty mangled up latches to get them working again, but it is not easy to do, and more difficult to explain with written words. I had the passenger side latch to use as a reference to see what a good one is supposed to look like (the passenger door often isn't used very much, so they aren't damaged like the drivers side).
Carl
