Vokera combi - falling water pressure

Every time the mains water supply to my Volkera Linea Max combi boiler is shut off, the domestic hot water (DHW) fails.
Before the shut off the DHW is ok, after the mains supply is restored the DHW is reduced to a trickle (1 litre per min). Over the next 1-2 weeks the pressure increases back to normal.
Does anybody have any idea why this may be happening?
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Possibly an airlock that takes time to clear?
Christian.
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