I would need the model number of the pump to be sure, but the only 2.5 HP I see will max out at about 65 PSI. Probably 67-68 with flooded suction as when pumping from an above ground tank. So if it is maxed out at 67 PSI, it is not pumping any water. The more water you use, the lower the pressure the pump can build. At 50 PSI, that pump can put out about 40 GPM.
If you want more pressure, you will have to get a pump that will produce more pressure and less gallons. Either a multistage centrifugal pump or a submersible placed inside the storage tank will give you more pressure.