pressure problems with halstead ace boiler?

since putting the heating back on after the summer i have started having problems with my halstead ace boiler. the pressure keeps dropping. i have checked for leaks but cannot find any. i have topped up the pressure to between 1-1.5 but within a couple of days it has dropped below 1 again. i have turned the boiler off completely and re-started it but when i checked it again this morning, although it hasnt dropped as much it is still slightly lower-everything else seems to be working normal.