It depends on the cause of the problem.
Many times this occurs because the latch no longer lines up correctly with the strikeplate on the door jamb. (The hinges can sag, or the house can settle a bit, pulling the jamb just out of line, etc.) If so, then the parts need to be realigned.
Some storm doors have adjustments, but many times the problem can be solved by shimming one of the hinges. (A shim is a thin piece of wood; it can be placed underneath a hinge, raising it slightly.) Alternately, the strikeplate can sometimes be raised or lowered just a bit.
You can see photos of some of the parts mentioned in my Hub on replacing a door (which is another possible fix!)