Why does the pump on a pumped accumulator vessel not always activate when I turn on the tap or shower?

The pump on an AccuBoost accumulator turns on when it recognises the system pressure has dropped below its activation threshold. The minimum pressure to activate the pump on an AccuBoost accumulator is 1.4 bar.

When an outlet, such as a shower or tap, is turned on the system pressure decreases. So if only a small amount of water is used, the pressure may not decrease by enough to reach the minimum pressure required to activate the pump.