It's going to be process of elimination. Start right at the block and see if water is coming out of one of the two hoses. Generally, the 5/8 hose is the water to the heater and the 1/2 hose is the return (keeps the water in the heater longer) If you have flow out of there, then take the hose off at the heater and see if it is coming through. Some have a shut off valve in line, make sure water goes through that. Water usually goes in the bottom of the heater core and comes out the top, but not always.

If you have water going into the heater, then make sure it is coming out, could be a plugged up heater core. Then simply make sure you have the correct port for water to reenter the block. If you have it hooked up where it is pressurized it will not make the circuit. There is no magic to this system, it just siphons regular coolant off of the engine and diverts it through the heater core. Very simple arrangement.

Don