There are a couple of things you should check. Starting with a cold engine, monitor the temperature of both heater hoses near the firewall area after the engine has been started.  Care should be taken since the plastic ends are known to become extremely brittle and can snap off if disturbed. You will be feeling to see if both hoses warm up equally and in a reasonable amount of time. Five to ten minutes with the blower off. If there is no build up of heat or if it is limited to one hose there is a coolant flow issue. Either the coolant level is low or the coolant volume/flow is low. A restricted heater core would be most likely. If both hoses seem to heat up well and evenly, set the system to full heat and high blower. Monitor the hoses to see if one of the hoses start cooling down. If so there again is a coolant level or volume problem.

The other thing to check is the operation of the temperature door actuator. You should be able to see the door shaft rotate in a 90 degree arc as the temperature setting is changed from full cold to full hot. You can take this test further by removing the actuator and physically turning the door with your fingers. You will be looking for a temperature change as you turn the door and feeling for s distinct thud as the door reaches the end of its travel. The door has to be moved quickly and with some force to feel/hear the thud. Do not use anything other than your fingers to turn the door or damage may result.