DHW pumped system

[Marnhullman]

I have a new build house with pumps on both the C/H and DWH.

The DHW pump only runs when the hot water is being heated via the timer, which is fine. Recently the hot water flow in the upstairs taps is very slow when the taps are opened - this during the 'timed period' but does improve on subsequent use, during the timed period.

Any thoughts as to why this should be e.g. is there some sort of sensor that is restricting the flow unnecessarily?

Thanks