My 98 driver and sometimes driver side rear door do this...what has happened is the little "rubber" eschucion [grommet or whatever] has been sun damaged over the years and is swolen and a bit sticky--especially when hot outside it provides to much resistance to the lock "knob" for it to unlock...you can remove the grommet and see if it works properly then. You will know if this is the problem or if the lock power solenoid is the problem.

I purchased new grommets but have not installed as the cool weather hardens and shrinks the grommets and the locks work fine.