drivers side side door closes but does not latch then opens again...how do you adjust the locking pressure

First: lubricate the lock with some graphite compound (sold in every parts store). Second: The hinges may be sagging, you can buy door hinge replacement bushings/pins from any auto parts store. Be forewarned that while this is a straightforward job, it is a PITA. Once done your door will once again be lined up properly with the striker on the jamb. Good luck.