It may be possible that the motor in your window regulator is actually working to drop the window, but the cable in the window regulator has too much slack in it to actually lower the window. You can test this by opening the door and then pushing down on the glass. If it easily can be pushed down about an inch or more and has a ton of play allowing you to move it up and down, this could be your problem. The good news is that it is super easy to fix it for about $3.00.
See this thread:
http://986forum.com/forums/general-discussions/25704-window-regulator-permanent-fix-stretched-cable-3.html