Could be that the heater core is bad/clogged...Also check the hoses going
and coming from the heater core...

IYM

Have you ever had the cooling system flushed? Sometimes heavy crud settles
to the bottom of the radiator and heater core and impedes flow.
Feel the hoses from / to the heater core while the engine is hot, heat on
full and fan on high. You should be able to feel a noticeable temperature
difference between the inlet and outlet hose temperature - if not, heat is
not being transferred from the core to air in which case it may be a problem
with the air control door position.

Likely causes:
1. Low on Engine Coolant

2. Stuck heater valve (some vehicles have these near the firewall and stop flow to heater when set to cool or A/C is used)

3.. Thermostat stuck open

4. Collapsed hose (can happen if old - no coolant flow)

5. Blocked heater Core

6. Defective/Failing Water Pump

Maybe the little knob that adjusts the temperature door is not
moving the door all the way to hot.
Fred
94 SC2 with lots of heat