I fixed mine...
Mine did this too. Top latch would work, bottom one wouldn't. Pulled everything off the door. Found rod had gotten stretched. For some reason it was too long. When you pulled the latch it wasn't long enough to pull the actuator on the bottom to release it. I just bent it a little to shorten it up. Works like a champ.
Lessoned learned. You don't really need to pull the whole inside door cover off. It does however make it ieasier to see. You can access the rod through the vent on the back of the door. The vent is on the part of the door that faces the bed when closed. Pick up or push down on it. I though I was going to break mine getting it out. I don't think you really could though. It's got a tab on one side, the bottom I think, and hooks on the other. If this helps when you put it back in you have to put the bottom in first, the tab, and then the top will snap in to place. With the vent out you can stick your arm in and stick your arm all the way to the front of the door. You'll feel the rod that runs from the bottom latch to the handle. Grab it with you hand and push it with your thumb to put a little bend in it. Tightens it right up. If you get it too tight, you'll have the reverse problem. The bottom will unlatch before the top.
To verify the problem before you go fixing it. With the door open, close the bottom latch. you can then reach in and pull the rod up, towards the top of the truck, and see if it unlatches. Mine had a lot of slack. On mine I had to really pull to get it to unlatch. I had to pull the rod as far and about as hard as I could in the position I was in to get it to unlatch. After putting a little bend in it. You could easily pull it and it would unlatch.
Don't know if this is the right way to fix it, there might be a better way. Mine did this about 6 months ago and my fix is still working!
Good luck,