Had the same problem with my '05 driver's rear window. As recommended by some old timers on the board, a good hard slam of the door fixed the problem. Don't know why, it's been about a year ago.
When I had my '01 I had the same problem on the same door. It turned out to be the "in-track" window shade causing the problem. It would swell in the heat and cause the window to stick closed.