The water must be going somewhere to drop the pressure. Check around the rad valves when the system is cold. It could also be that the installer failed to make a joint properly, it happens more than you would think. if it's not the valve connections and you haven't got any unexplained damp patches in the house you will have to pressure test to isolate the leak. This will involve testing by temporarily installing valves and pressurising individual pipe runs. Bit of a nightmare. Look hard for damp.