boiler pressure?

I have a Halstead Ace High Combi boiler. The pressure in the boiler keeps dropping below 0.5bar then it shuts off. When i open the valve below the boiler i must be repressurising the boiler with water. I just watch the pressure gauge change as i open the valve, until it reaches 1.2bar, but it still does not come on. I was told to bleed the radiators which i did and it came back on for a few hours, then the same happened again, the pressure dropped and it went off again.I have checked for leaks and dont have any. can anybody help me with this problem