Vokera 25he pressure issues

I have a problem with my Vokera 25he boiler losing pressure when the central heating warms up.
The pressure sits steady at 1.5 until the radiators reach temp and then drops to zero within the hour. I have 5 radiators on the ground floor and 2 on the first.
One radiator on the first floor has air in it each time the pressure drops. The rest are fine.
The problem started over 1 week ago and I have bled and repressurised the system several times and can find no sign of a leak. Checked the prv and the pressure is fine there. There is no pressure loss when the system isn't used.
I have isolated the boiler and no pressure drop occurred meaning the problem must be in the heating pipework. Is there a way of finding a leak without pulling up floorboards?

 

Does the pressure increase when in use? If the rad on ff drops I'd say a leak on rad below on gf or hydrogen trapped.

 

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