When your door is not catching the strike plate correctly it will not shut properly. The first thing you will want to do is to check the alignment  both horizontally and vertically.  Vertical alignment is easy to check from the outside of the door because it is easy to see how everything lines up as you are closing the door. If it is lining up vertically, the problem is that it’s set too far out or two far in. You will need to adjust where the strike plate is so that everything will line up properly. You will need to start by removing the strike plate. To do this you will need to remove the two screws holding the strike plate to the door frame. You will than take a chisel and remove material along the edge to move the strike plate in whatever direction necessary.  Once all excess material is removed you can drill plot holes for the screws in the new position of the strike plate to aid in installation.  The final step is to reinstall your strike plate in the new position.  This will solve your problem and your door should shut properly with ease.