equalize pressure in combi boiler?

Just had a new combi boiler fitted. But the pressure in my bolier keeps dropping and my radiotors keep filling up with air. At present I need to 'bleed' my rads and equalise the pressure in my bolier at least once a week. This can't be right. it has been suggested that I have a leak somewhere. I can't find one. Although I have notice a 'damp' patch on one of my external walls that wasn't there before. My boiler is a worcester junior 25i. Does anyone have a theory or suggestion? Many thanks.