I had the same problem a year ago. You just wouldn't expect the rear door latches to act up on a low mileage vehicle with almost no passengers, but hey I'm getting use to it.
I spent nearly an hour yanking and pulling and eventually partially removing the interior panel of my pass. side rear door. I could not find any posts with enough info to fix it. I decided to tear into it. OMG! You have to cut the steel support panel, under the interior panel, off to access the lock mechanism. I spent hours doing that. I can't remember all the steps (still trying to forget ) The outer button has a pinch point on the actuating cable that starts slipping and doesn't quite pull the cable. There are instructions on the Internet for lock disassembly. I fixed mine by adjusting the cable grip but it still occasionally acts up. The only permanent fix seems to be a replacement of the actuator. I can tell you it is critical to "set" the latch cable inside the door exactly as the instructions say. I found out the hard way and had to tear the door back apart after the first lock sequence. Luckily, I made it more accessible for future repairs.
I'll check if I have the info saved.