the two pipes going to your bulkhead - connected to your new matrix - if these have alloy coloured "valves" on them (mounted about 3 inches up the pipework) they could be blocked. these valves are big and you cant miss them. there will be a small "bypass" pipe joing the two valves.
if the pipes have no valves on them then... i guess you could have an air lock? in which case you will need to bleed the coolant.
i assume you connected all the heater control cables back up?
