The system is losing water somewhere or the pressure release valve is dumping water due to too much expansion of water.
First isolate the central heating from the boiler by closing the CH flow and return, located just under the boiler. Then pressurise system and run boiler. If you still loose...
Check inside the boiler for any signs of a leakage. The water can be lost by leakage or evaporation. Check the CH and DHW diaphragms as the diaphragms have to be flexible to operate the electrical contacts and non porus to prevent fluid loss. Do not change components without good cause and proof.
Yes I have the same problem and have changed the PRV, the pressure guage and pumped up the pressure vessel. Nothing changed. Then on close inspection noticed a lot of scale around DHW valve and traces of water near plunger that operates the electrical contacts. I then concluded that the DHW...