The heater is 240V and is on a 20A breaker with a total of 4 heaters cascaded and this one is the last one. When I test the voltage at the problem heater I get 115 V from white to ground, 115 V from black to ground and 0 V from white to black
Could also be one of two things
Your testing shows that the problem heater is either miswired to the heater preceding it or it is not on the same circuit and has both the heater hots connected to the same leg.
0 volts between white and black at the heater that is not working is telling you that the black and white are on the same leg of the service. They need to be on opposite legs. Thats what the double pole breaker does. If that heater is indeed supplied from the heater preceding it then you have a miswire at the working heater. Your black and
white both have been connected to either the black wire or the white wire at that heater. They need to be connected black to black and white to white not black and white to black or black and white to white.