If you have an exterior door that requires a shoulder to force it to latch, you may want to spend a little time to fix it. It likely is a simple fix.
There are many possibilities why a door won’t latch starting with the latch itself. Make sure the latch is aligned with the hole in the strike plate. If it is not, has the door sagged? Hold the door knob in your hands and try to lift it up. If there is movement, you need to correct it. You can use long deck screws in place of the hinge screws on the jamb side of the top hinge. If the door is firm, you can either move the strike plate up or down, or even file away a little of the strike plate to allow it to receive the latch.
If the latch contacts the strike plate but it tough to get in the last little bit, the weatherstrip may be to blame. Something is obstructing the closing of the door, and it is often the weather strip around the perimeter of the door. It may be that the wrong type was used, or that it wasn’t installed correctly, but that is a good place to start. Check both the hinge side and the latch side for problems.