Probably an air lock in the matrix that's stopping the hot coolant from flowing through it, (especially if you've just fitted/ connected up the heater). With the heater valve open, try disconnecting the inlet/outlet heater hoses on the engine side of the bulk head, and connecting a garden hose to one, and a spare length of hose leading to a bucket to the other. Then gently turn the garden tap on and flush the matrix until the water runs clear into the bucket. This should prime the heater circuit with water and purge it of any air. Then reconnect the hoses and run the engine at idle until it gets up to temperature, it helps to do this with the rad cap off, so that any remaining air in the system can escape. Don't turn the garden tap on fully though, as full mains water pressure could damage the matrix.