1 pole breaker with 2 slots for wires to heater (red and black wire) aiw 12 wire. At heater red to red and black to black, bare ground to green screw. Test light to both leads and nutral lite. Why not working.

If you are using a tandem breaker (one breaker that has two switches and fits in the space of one standard breaker) you are only getting 110 volts, not 220. You need a two pole breaker, if the breaker is a tandem as described above.