How can I fill system pressure loop?

I have MAIN COMBI 24 HE boiler. When I dont use it, pressure stay in 0 and when I use hot water it goes up to 1 and when I use gas it goes up to .5 very slowly. I open the valve of soft pipe which is connected the main pipe, water pipe, nothing happens. I believe that is the filling loop. Im pretty sure there is no leakage. I have no idea what to do.