I have a 2000 RX. I had mine off recently so I could repaint the cross-bars, which had peeling paint.
Those little ***** screw into cleats that ride in the track below. If they've not been moved in a long time, dirt and crud can build up in there and they can get stuck in place. Loosen the ***** and give the assembly a rap with your hand to break the cleat loose. Even once they're broken free, they don't glide very well, and you'll need to go side to side and move the cross-bar to it's desired postion.