Air lock ?
Do you have air in the heater core. some heater cores are higher than the engine. and need to be bled . If so loosen the upper heater hose, slide it back aways , Drill a 1/8 in hole slide the hose back in position snug the clamp. start the engine, warm it up then loosen the clamp and slide the hose to uncover the bleed hole and let any air out , then reposition the hose and tighten the clamp.