The door button could be binding in the door panel. Sometimes plastic parts warp from the heat & bind. The back of the button might also have become dislodged from a linkage rod directly behind the door panel.
Might also be linkage inside the door binding, or there are some broken pieces. Door lock solenoid may also be sticking too.
Try some spray lube around the botton and door lock mechanism and see if it helps.
========= Harryface =========
1991 Pontiac Bonneville LE ~_~_~266,400 miles_~_~~_