Like pawoSD, I suspect it is drawing in outside air when it shouldn't. Read the Owner's Manual since the temp controls are non-intuitive, though the problem is likely not user-error.

As mentioned, in the engine bay, feel the tubes going into and out of the heater core. If both are warm/hot, the problem is in the cabin in the air box (flappers doors, controller). If they stay cold, and the mono-valve is unplugged, and you rev the engine a bit (water pump doesn't cirulate much at idle), the problem may be a clog, either in the heater core or in the electric auxiliary pump blades (near alternator). I removed mine (leaking shaft seal) and replaced with heater hose. I get plenty of heat (when the controller cooperates), just not much at idle.