Depending on which type heater you have, you may have a bad heater valve. I installed a refurbed valve when I installed my Deluxe fresh air heater and learned that when it is in the "off" position (cooler rather than warmer direction) the hoses will not fill with water/coolant and therefore the core will not fill and become hot.

Check out your heater valve or turn your valve to the open position.

Spot

Chuck
1950 Chevy 1/2 ton (all original)
1951 Chevy 1/2 ton (future streetrod)
1941 Chevy coupe
1938 Chevy coupe streetrod