how can i correct central heating pressure?

i have one radiator which is half way warm and half cold so i decided to bleed. i switched off the boiler and bleed the radiator. when im done i decided to go through the rest of the radiators and bleed all as well. then i switched on the boiler manually because it was set on timer before. however when i checked the radiators after more than two hours they are still cold. How could i correct the central heating pressure? Can you please give me a detailed procedure on how to it? Greatly appreciated.