You both may be looking in the wrong direction.

I have a Cambo standard and rail in front of me. I've fiddled with the set screw on the bottom of the rail rider. It raises and lowers the friction drive roller and as far as I can see its major effect is to affect the ease of mounting the standard on the rail.

The key to your problem, Parker, and the one I created for myself by fiddling is friction drive. If drive roller slides against the rail it won't drive. Mine was a little cruddy, slipped. Cleaning it -- I held a paper tissue against it and turned it ... -- restored friction and its working as it should now. Will move the standard forwards and backwards on a horizontal rail, can't move the standard upwards against gravity on a vertical rail.

Clean your roller and rail, Parker, and see what happens. I know it is counter-intuitive, but friction drive needs friction. No friction, no drive. There are two rollers inside the rail rider's upper section, so it doesn't slide on the rail. You might want to check that they turn freely. You don't have to defeat sliding friction, the bearings in the rail rider do that for you.

George, are you sure you know which knob is the drive knob and which is the locking knob? I know my question looks insulting, but I can't reproduce your problem so it has to be asked. The drive knob is on the left side, the locking knob on the right; the locking knob backs way out, the drive knob doesn't.

Good luck, have fun,