How do I determine why a sealed central heating system is losing pressure?
Check the expansion bladder has correct precharge pressure: with boiler barometer at 0bar air pressure inside the bladder should be 1 bar.
Also check the automatic de-aerator are working properly (maybe thosae are clogged by limestone or just failed): if those fail to discharge air outside air could build up inside the system and act, de facto, as if there is no water.