Usually when your dead bolt is not engaging properly, it is most likely due to a misalignment of the lock and door frame. Wood-frame homes can settle and move over the years, shifting door frames so that deadbolts and strike plates get out of kilter.
In order to remedy this situation, tighten all the door hinges because loose screws can cause a door to hang down and miss their strike plate. If that doesn't work, you might try making adjustments to your strike plate to make the bolt fit again. To do this, you should first rub the bolt end with some chalk. Then turn the latch so that it leaves a chalk mark on the strike plate showing where the bolt hit. Remove the strike plate and use a metal file to widen the hole where the chalk was left. Now just put the strike plate back on and see if that worked. If not, you may need to do a little more filing.