If you have had no water in the system. You will have to go around and bleed each radiator until you get all the air out, you get water out of the radiator air bleed when its full.
Its a bit of a pain to do and you will have to go backwards and forwards to the boiler and refill it, just relax and take it easy, it can be a slow job.
When you have done the air bit, run the heating and make sure each rads getting hot. Start nearest rad to boiler and work away for both circulation check and air bleed. If you have trouble getting it to the furthest away rad, turn all the others off usually does it. Might need a bit of a balance but I would try air and circulation first.
The thinking is: The system is part full of air and to get it out you have to bleed each rad.
Could be as Newbie said as well.
Has anybody taken the radiators off the wall recently to decorate behind them and not filled the system after putting them back?
Very common problem on sealed systems I should think. Especially if people have lived in a house with an open vent self filling system. They probably figure central heating is central heating. Its an easy mistake to make I think.