What are the reasons a 98 silverado loses heat? on 1998 Chevrolet 1500 Pickup

Just recently my truck lost heat. I have checked and tested about everything possible and replaced a couple parts and still have no heat. At first i was told the switch was bad but it has a new one and i still dont understand why i have no heat. The blower motor, resister, fuses, switch and hoses and things are fine. Before it quit i had to leave heat on high and if i turned it down or off i couldnt get it back on.

It is possible that the heater core is clogged.