Oven or Constant Light
These are the most important objects of the printing. Heating the printing stamps and the hitting the anvil needs very hot oven or light to do that.
I don’t know if I understand right, but both anvil and stamp don’t need to be heated for stamping. All tre process is done with cold instruments.
I would have told you to do so.
Also, your washers look quite different from the one I used. I never saw a washer with borders. Are you sure are you using the right tool?