Once had an airlock in a Hillman imp. Engine at back heater matrix at front - nightmare.
Couple of suggestions.
1. squeeze hoses and listen - if you can hear "squelching" noises there is air in there = airlock.
2. Try carefully clamping the pump outlet pipe just after the tee off for the heater to force all the pump pressure thro heater. Do it cold when thermostat is closed.
3. you should check thermostat is working correctly - they do fail open sometimes i which case all the flow will go through the radiator reducing pressure for the heater.
4. lastly - unlikely - water pumps do loose efficiency with age if the blades are eroded.
Best of luck.
