Dave, based on what little info you posted, it's hard to tell. But, if by "high" you mean the full Hot setting, and assuming the blower fan works fine, then check the following: Does the Temp gauge show that the engine is reaching normal operating temps of 1/4 to 1/2? If so, then your thermostat is probably okay. Is there plenty of coolant in the resevoir tank? Simple, but sometimes overlooked. Is it an electronic type control where you can set the exact temp you want the cabin to hold? If not (and sometimes even if so) then start crawling under the dash to see if when the controls are activated, the cables actually move the valves on the blend box / air distribution box housing. The cables can slip in their clamps and the whole cable will slide, rather than just the steel core inside. This would prevent the outside / ac blend air door from closing one way or the other, and or prevent the valve which sends hot coolant to the heater core from opening. Thus, little to no hot air. It is a wilderness and a pain under the dash, but without crawling around under there, I couldn't answer your question for sure.
It might not cost alot to get a dash control unit out of a junkyard and replace yours, and you might discover cable problems in there while you have it out. Otherwise, it gets more detailed and expensive. I would not suspect a heater core, since when they fail, it usually involves coolant in the cabin floor. But if the dash unit doesn't do the trick, then you might start looking at the core, since while replacing it, you will be exposing many other parts and might find other problems that might be contributing. Good luck.