fixing a worcester boiler?

I have a worcerster bolier which is nearly 2 years old, the pressure bar keeps dropping down into the red section.I increase the presure back up to one n half which is what it normally runs at, but it drops back down within hours.I have checked for leaks, i have none, I have bled all the radiators.Istill have heating and hot water .Ijust need to know why the pressure keeps dropping and is it dangerous.