Radiators can become air locked some times after you've drainned a system either because of pipework or because you've not drainned it properly. Clearing the air locks are easy all you have to do is follow the simple steps below.
Put your programmer on heating only mode.
Turn off all radiators except the one affected.
You will hear the water moving in the radiator and it will start to get hot.
Leave to establish heat for ten minutes and vent any air collecting in radiator.
One by one turn on the other radiators starting from downstairs, checking that each one is working before moving to the next.
Pulling the water through is done by connecting a hose pipe to the radiator affected and draining off some of the water. When the water starts to come through turn off your hose pipe.
Another way is to isolate the radiator both sides then drain the radiator. Once drained undo the radiator valve at radiator connection then drain from each valve. This will also "pull through" the water. Again once hot stop and repeat with other valve. When both are okay connect back to radiator anf ill it up. The radiator should now be working.
In some cases you may have to isolate the d.h.w circuit by closing the valve on the circuit, usually located near the cylinder. don't forget to open it up once you've finished.
If not alredy it would be a good idea to balance your system.
In extreme cases you will have to pull the water through the radiator. (See below.)