Sticky doors

I am sure this has been answered before but I can't find anything solid...
1968 Dart 4 door, 3 of the 4 doors have varying levels of how much the door sticks closed. The best door opens and closes like new, and the door handle button sticks depressed on the worst door and wont pop out unless you give the door a good whack.
Question: Can I just re-grease the inside part of the latch? If so, how?
Or do I need to buy a whole set of new latches?

PS. the area around the strikers do not appear to be cracked/broken/bent so I am pretty sure its not the common broken striker problem