with combi boilers, you have to adjust the pressure every once in a while. The boiler should have adipslay on it sayign what the presure is (bars) it should be around 2.5 if it is not, this might be the probalem you are having? maybe? there whould be alike a tap thing under your boiler on the pipe feding into nit, twist this and it will add water into the system to build up pressure. this might only be for the heating though, i really dont know? just thought id mention it anyhow, as this is what happened to me, but with teh heating, it never used to reach the 2nd floor. Now it does :)