biasi prisma 24s DHW only luke warm

[JBseymour]

CH piping hot
Plate exchanger very clean defiantly not blocked
DHW flow switch 'actuator' diaphragm is sound (no splits) and the diverter valve is working correctly (stripped down and pressure checked -in DHW mode it doesn't leak back into CH circuit).

But... I need to turn the kitchen hot tap on very fast to get the flow switch to trigger the burner so the water is only luke warm.
If I turn the tap on slowly and manually trigger the flow switch, the water is still luke warm because the diverter valve is not fully closing.

Could the fault be with the two hydraulic pipes supplying pressure to the diaphragm (restricted maybe).
It used to work fine up until few months ago?

 

[JBseymour]

Turned out to be bad NRV on shower mixer that was letting cold water out of hot taps.
Boiler was only getting enough DHW flow to trigger flow switch when hot tap was fully open as a result.
Logical in hindsight.