Draining down sytem?

I drained down my system in order to clean the system with Fernox through the draincock on the boiler but it took well over an hour to empty the tank. After I put the Fernox in and filled the tank again the whole system seems air-locked and it is days before the heating gets through to some radiators even after leaving the air vent open. Do you think the problem is an airlock or some kind of blockage and if so how can I fix it. Many thanks.