When the engine is fully warmed up, grab the heater hoses and see if they feel warm or not. If they are both warm, fluid is circulating through the heater core-maybe not as much as you need, but probably a door not opening if both are hot. If the inlet is hot and the outlet cold, its not getting through the blocked heater core. If both are cold, hot water is not getting to the core, so suspect low coolant level, air bubble, or even a water shut off valve not opening. That's a good start anyway.