It’s possible he’s freaking out and forgets he has the Force in his encounter with the Rancor. I know canon says Rancors are not native to Tatooine, but I used to imagine he’s facing his greatest fear, the monster he used to hear echoing across the desert. Now he’s trapped in a room with it, and for a moment forgets everything he’s learned.
However, I feel that it would be worthwhile for this edit to attempt to make Luke throw the skull or hit the button with the Force.