Hi Steve and William,
These are nice warmers. From your description you may be putting the pallet in backwards Steve. I may be reading it wrong though. In any case if it works then go for it!
The stones would normally rest against the adjustable arms. The space or slot down the middle allows for separate heating of the pallets in case you just want to adjust one stone. William’s right on about the adjustment. You can adjust the stones and use the scale at the other end of the arms for reference.