Often, with a newly filled system, there can be air in the water used. Over time this is released into the system causing a pressure drop. Another cause of pressure loss could be not having enough expansion in the system. There could also be a slight leak within the system which may not be noticeable without detailed checking. Finally, it could be due to a component failure within the boiler. If the system requires topping up frequently it needs to be checked by a professional to find the source of the pressure loss.