why won't heating work now i've built up the pressure in biasi 24s?

i have a biasi 24s combi boiler. the pressure in the boiler has gradually dropped over the last 12 months until yesterday when i lost both hot water and central heating. i have been told how to build up the pressure to a little over 2 bar and i now have hot water again by the looks of it but i still have no heating. the flame keeps flaring up periodically but then goes straight back out. if its any help, 3 radiators out of the 5 downstairs stopped getting warm during the summer (i guess this might have been down to low pressure maybe?) how can i get them all to work again