Daikin Heat Pump not working in summer?

[climbinggeorge]

I have a daikin altherma 3 split system and a mixergy hot water tank.
My understanding is that the mixergy tank is the 'smart' part, this calls for the hot water as required.
I set the schedule on mixergy to heat 90% of the tank at 5am, and then if it drops beneath 30% to reheat to 75%.

Except, it's not happening.
I can see on the mixergy side it's calling for hot water, but it doesn't react immediately and can take hours before suddenly sending hot water in.
If I turn the ashp off and on, it will react to the call and heat immediately.

This hasn't happened in winter, so I presume it's something related to hot weather, perhaps not wanting to run a short cycle?? any ideas welcome as my partner keeps managing to have cold showers