why ami losing pressure on my green star 28i junior boiler?

system and dhw working ok! check radiators for leaks and all topped up. The system is not dumping water ive checked! even at 3 bar.I have had a plumkber top up expansion vessel pressure but still over 5 hrs bar presure goes from over 2 to 1 then to 3/4 in the red. presure is constant while system is off. I have a sheared of valve key in the cold water loop thats in the open position would this affect it? The presure started dropping some months ago but miniscule now i am topping it up nearly every day or other day.