The gear type door latches are prone to sticking if they are "over engaged" for the sake of a better term. The gear on the door is being displaced upward into the fixed teeth on the pillar. This can happen if the car is twisted slightly on an uneven parking surface. My '57 does this on occasion and I will move the car a little to release the door. Sometimes just straightening the wheels will do it. I had a '56 Ford convertible "back in the day" that was horrible for this.