My TankBoost is working but losing some static pressure over time, why is that?

This is not a problem for the TankBoost, and we would expect this during the normal operation of the unit. There is likely a few reasons for this, none of which will cause an issue with the unit or for the system. The most likely causes of this would be a small weep on an outlet due to the higher pressure such as a tap not fully closed, or a valve in a toilet cistern allowing a very small dribble into the cistern until it has fully filled and then the pressure will hold. The other likely reason for the drop in pressure over time is contraction in the water within the pipework as the hot water from the boiler held within the pipes gradually cools over time and reduces in pressure.