Mixed battery performance

I have an 8.2kWh battery installed August 2022 which has provided good performance, typically discharging up to or close to rated energy when discharged even at 3kW. Recently I added a new 8.5kWh battery and they don't play as well together as I expected.
The 8.2 typically discharges faster than the 9.5 although of course the indicated DC voltages are nearly identical.
If I discharge them at 3kW then after about 2.5 hours the discharge is throttled to 2kW and soon after it might drop even lower, sometimes going into an unstable state where the discharge rate fluctuates up and down.
If I reduce discharge to my house load (Eco mode) then it will stabilkise for a while and nbehave as expected until the 8.2 reaches about 30% SOC (9.5 now at 45% SOC) at which point discharge reduces to 100W.
Charging seems unroblematic althought 8.2 seems to stop at 95% ((.5 then at 100%)
What's going on? How can I get more energy out of my batteries?

Suggest you contact your installer in the first instance if you’ve had the 9.5 recently fitted.

Presumably the 9.5 was wired as the primary battery and the 8.2 as the slave? The larger battery should be first in the battery chain and the first battery usually takes the load and charging first so strange that you report the 8.2 discharges faster.

Are you on the latest inverter firmware and battery BMS software ?

geoffreycoan yes, the 9.5 is designated primary, the 8.2 is designated secondary. I will have to get a look at the wiring inside the trunking to confirm the battery chain.
AC3 inverter software is D0.212-A0.212
battery firmware is 4009 & 3020

Thanks