If the pressure definately doesn't rise
and there's no leaks from the pressure relief discharge pipe then it points to a leak somewhere in the system.
Is this a condensing boiler ?
The secondary heat-ex sometimes splits and you can lose the pressure via the condensate discharge pipework. Hard to diagnose though - as when the boiler is on there'll be water coming from the exhaust gases.
If the boiler is under warranty - get the manufacturers out
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