Did you check the heater for clogging?
Check the temperature blend door for sticking and try adjusting the cable ( if equipped. If automatic HVAC the blend door could be electrically controlled)

TEMPERATURE BLEND DOOR CABLE

Set temperature blend control lever to COLD position. Disconnect cable housing from retainer on heater case (leave cable wire connected to temperature blend door lever). Push cable in direction of COLD position until temperature blend door stops. See Fig. 1 . Attach cable housing to clip. Ensure control lever moves between COLD and HOT positions.