We would check that the (hot coolant) is actually getting to the heater matrix. First off, check at the heater control valve. The heater control valve is usually in line with the inlet hose of the heater core. The exact location of the valve may vary by vehicle, but it's often near the firewall. Check that you have hot coolant coming to the valve and then coming out from the valve. If you only have incoming and no outgoing then the valve has failed and as such no hot is getting to the matrix.