I've had to deal with symptoms like this recently at the shop where i work. First like another person says, feel the heater core hoses (which you will find under the hood. there will be two hoses that run into the fire wall.) If both are hot, then you have circulation and is probably not the heater core. Also, if you have an automatic flap for your hot or cold side, it is possible that the flap is stuck, or just not getting the electrical current to tell it to switch from hot to cold. Sometimes, and i've seen it happen, If you give the dash a few smacks, it will loosen up the wedged flap and will allow it to move freely. Also one other thing, check to make sure that you have plenty of coolant in your radiator, cause of course that hot fluid is what allow you to have heat. And one final thing, if your vehicle has a coolant system bleeder valve, you may want to bleed the coolant system. That may also be your problem. Hope this helps at all.