Since the day I bought our GMC, the door has closed hard with only catching the first latch. I thought I had done everything to the door to correct the issue, but slamming the door was the only way I could get it to close and hold at only the 1st latch. This thread gave me the impetus to try again. So with the help of Paul Doane who has helped himself and several others correct the issue, he started by putting a piece of paper between the door and the frame, closing the door and puling the paper out noting the resistance. We found the paper test to be quite tight at latch/striker plate. We started then by moving the striker plate toward the outside as far as it would go. Still would not go to the second catch. So, we took the door seal off. Using Lacquer thinner in a finger tip spray bottle, we easily pulled the whole seal off without damaging it. Without the door seal, the door closes easily to the second latch. What I then found out is that there are two styles of door seal. The original which is a pretty thin flat rubber seal and replacement seals that are much thicker and hollow to allow for compression sealing. So, at the hinge where there is a lot of space between the door and the door frame, I put the old full thickness seal back in, but for the latch side of the door, I cut the old seal in half down the length, removing the hollow, and then glued the seal in place with contact cement. Now the door closes to the second latch with a normal push. The hinge seal is a little tight, but I figure after a couple of months use, it will eventually get a compressed set allowing the door to close even easier. My striker needed to be adjusted as far outward as possible and centered up and down with the latch so that the both hooks of the latch center around the striker post. When I get home, (we are south for the winter) I am going to try the original door seal all around the door. I'm told that a original seal is much more expensive than replacement seals and may be why some of us have, because of price decided to try the replacement style, not knowing that it might be causing the slamming and only single latching.
So, that's my experience. Hope it helps someone.