Not getting warm

First thing to check would be the coolant level. If the system is slightly low, no coolant will flow through the heater core. Could also be a bad (open) thermostat or a clogged heater core. When was the last time the cooling system was flushed & refilled?

If it was low on coolant wouldn't be getting hot?

Note I said 'slightly low'. There may be enough to cool the engine (especially if it is cold outside) but not enough to flow through the heater core to heat the interior.