Okay, you close it, it goes to the fully closed position and lines up flush with the rest of the body and will not stay there or you close it and it stops about 1/2 inch of closing and does not set flush?
For the first one, the door latch spring is broken or the latch pawl is binding from rust or dirt.
For the second one look into the opening on the door where the striker slides into the latch pawl. If you see a piece blocking the slot (like the picture) the latch tripped closed. To open it you get a screwdriver or similar tool and put it into the hole (orange area) then "open" the door while pulling in toward the open slot to move the pawl open. In this case the pawl opening spring is binding.
The first thing to try for both would be to spray some cleaner/de-greaser into that area of the latch. I use carburetor cleaner or gun cleaner. Keep working the pawl open/closed until it moves freely and snaps open and stays closed. Then I spray them with graphite dry lube.
If the latch does not seem to work even after cleaning it is likely the springs are broken and latch replacement will be required.
Images (Click to make bigger)
Sunday, September 10th, 2017 AT 9:22 PM