Approx two years ago my boiler system was replaced with a combi type. Everything fine but one radiator in the system last year did not heat up despite bleeding it etc. I had to get a plumber who said the system needed balancing which he did and the radiator worked OK but only for a week or so. I never got the plumber back as this radiator was in a spare room. This year the central heating system has been turned on and this same radiator is cold but also another one, (both are upstairs as is the boiler), I have bleed them, but both were full. I have tried with all valves open on the other radiators and all valves closed but I cannot get any hot water feed to these two radiators. The hot water feed in pipe to the original radiator is just about warm but the feed to the second one remains stone cold. Is it possible there is a blockage somewhere?
If balancing the rad.with just IRV's ( bathroom or hall location common), try continuing balancing the rest of your rads. to put pressure onto the cool rads. If that doesn't work, then it is possible that your system needs flushing/rinsing to remove flushing chemicals/then refilling with a fresh charge of plain water dosed with corrosion-inhibitor. Unfortunately, it is not unusual for new systems to be installed without the above commissioning procedure being carried out, as it is so time-consuming.
Cleanser needs to be flushed out until all rads. drain out clear, as the chemical automatically becomes neutral after a few weeks and then all the flux & oil etc. taken into solution by it starts to precipitate out. Anti-corrosion additive is essential, especially now that aluminium is being used in boilers, and needs to be kept up to strength ;--- have it checked at the annual service.
I can personally vouch for this cowboy attitude existing in the trade, as my brand new system was left 10 days with plain water in it, and after complaining, they put both cleanser and inhibitor in together. They really do think the public are fools. Only a telephone call to the manufacturer of the chemicals clarified the required procedure. They are being recalled to flush the system fully, and recharge with inhibitor-doped water.