Central heating does not reach all radiators?

Profile potterton boiler 60e.We have 8 radiators +1 towel rail fitted in our house.The problem is when heating and water are both running only 7 of the radiators work.When only the heating is on they all work with the exception of the radiator in the conservatory the heat gets as far as the radiator, but that's as far as it will go.this was all working okay last year.The only change we've done is the bathroom where we removed a radiator when we had new suites fitted in the bathroom and down stairs cloakroom.All thermostats have been checked and duff ones replaced ,system has been flushed through and system bled.Even when we did a test with all the thermostats shut except for the radiator in the conservatory,it still does'nt heat the radiator Can anyone throw any remedies