To fix a car door that won't shut, force the door clasp into the open position with a screwdriver or other long, narrow object while the handle is held open. Most car doors fail to shut when the door clasp has been forced into the closed position and will not respond to the inner or outer handle.
The door clasp is located on the inner edge of the door panel, and is shaped like a small hook. When one of the handles is pulled into the open position, the clasp should move so that it catches on the corresponding part on the car itself. If it doesn't, this is the source of the problem.
Step 1: Fix the door clasp
With the door open, pull the outer door handle and keep it in the open position. For larger vehicles, ask a friend to hold it steady. Slide the end of a screwdriver or other long object against the clasp to push it back into the open position.
Step 2: Test the door
After removing the screwdriver, release the door handle. Shut the door to verify that the clasp catches upon closing. Repeat the process until the clasp opens and closes on its own. If this does not fix the problem, the vehicle might require the attention of a mechanic.