Wood-frame houses tend to settle and shift over time, which can knock door frames out of square. As a result, it might become tough to lock a dead bolt because it no longer lines up with the strike plate. If you don't fix the problem, the misalignment will worsen.
First, make sure all the hinges are tight. A loose screw or two can cause a door to sag and the dead bolt to miss its mark. If that doesn't solve the problem, try this:
Open the door and turn the dead bolt latch. Smear a little chalk onto the bolt end (above left). Turn the latch to draw in the bolt, close the door and turn the latch again so the bolt hits the strike plate. Open the door and look for a telltale kiss on the strike plate.
Unscrew the strike plate from the door jamb, clamp it in a vise, then file it down to the chalk mark (above right). Reinstall the plate and test the dead bolt. If it's still not locking, file away a little more of the strike plate until the bolt locks properly.