Every now and again, I find a heater core that gets a restriction and causes the car to not have heat or not enough heat. This is not the first thing I check for; in fact, it's pretty low on the list of things to check. Heater cores don't normally clog up on their own. It's usually caused by an outside force.
Before I get too deep here, a heater core is like a small radiator located inside your dashboard. Coolant from the engine passes through this and a fan blows air across it. The warm air you feel from the HVAC vents is a result of this process. If there is a problem with the heater core or cooling system, it will affect heater performance.
One of the main causes is not using stop-leak products properly. If you've had a coolant leak that you've attempted to fix with stop-leak and you have no heat or reduced heat, you might have a blockage in the heater core causing the problem.
Another cause is corrosion. If you run straight water in your cooling system or your cooling system is full of scale and rust, your heater core can clog up. While the engine is cool, remove the radiator cap and inspect the coolant. If it's a nice color (could be green, blue, orange, or perhaps even yellow) and free of contaminants, the heater core is not the first place I would look. However, if it's full of rust or looks brown or black, you might have other issues causing problems with the heater core. If you do see these contaminants, you might consider flushing the cooling system as well as the heater core to prevent issues in the future.
A good test to see if your heater core is clogged is to feel the hoses going into it in the firewall. Look at the firewall or bulkhead for two hoses close together. These are normally the feed and return hoses for the heater core. With the engine warm and running, feel the hoses. If the heater core is working properly, both hoses should be warm or hot to the touch; one might be slightly cooler than the other, but not by much. If one hose is hot and the other cool or cold to the touch, then you might have a clogged heater core. If that's the case, try flushing it out. Sometimes you can force compressed air and cleaner through a heater core to clear the blockage. I would suggest you try this first before attempting to replace a heater core. The heater core is part of the HVAC system in your dash. It might take a great deal of work to replace it, so try to clear a blockage before you attempt a replacement. Here's a video showing the process.
Everyone always asks me what cleaner I used in that video. It was CLR. It worked pretty well.
One last note: Jeeps are famous for clogging heater cores. I don't know why, but I hear about it a lot.