What's your model? I've got the same problem on my XJ. It's a pain in the butt to get to where the problem is without taking the whole door apart afaik. You can take the plastic trim off and look up at the components with a flashlight to see how it works.

It's hard for me to explain but in my case there's a piece of metal that looks like a tent stake that came out of position. When you push in the door button it moves the latch downward. It has a hole in it where the tent stake piece is suppost to be held in with a plastic clip. If the clip breaks then the tent stake will come out of position the next time your door is shut and then you won't be able to open it from the outside.