This is a very common problem that I see often on pressurised systems. On old systems with tanks in the loft you dont get this issue because they automatically fill when pressure or water is lost. Low boiler pressure on new systems can occur for many reasons:
1. Either there is a leak on the system somewhere, or the pressure relief valve (PRV) is faulty and throws water out
2. The expansion vessel has lost it’s charge and pressure rises when the system is hot & throws the water out again and when system cools the pressure reduces
3. You bleed a radiator and consequently lower the pressure without realising
4. Or simply the air molecules in the water break down over time and a small top up is required from time to time (usually once or twice a year).