So you should be able to grab the edge of one of the wheels and rotate it to turn that axis (the cursor hand will turn sideways to indicate you're hovering it). There's a limit to how much you can rotate it left or right though, so that's probably what happened when it got "stuck" (it should still be able to turn the other way). Turning the blue one away from where the camera is now should be enough to move the marble to the other side of the disc.