If locking your garage door has become increasingly difficult, it's probably because the locking bars are misaligned. When the door handle is rotated to the locked position, the two horizontal steel bars slide out in opposite directions and pass through slots in the door track. When the bars are misaligned, they hit the track and the lock doesn't catch.
The solution is to locate the two L-shaped guide brackets mounted to the door near the outer end of each locking bar. Loosen the screws on one bracket and use a hammer to tap it either up or down until the bar is aligned with the center of the slot. Tighten the screws and then readjust the other guide bracket.