Why is pool pressure low?

Don't possibly know why you could be loosing pressure on the pool. However you may be loosing pressure in the line to the pool from the filter. If you are also loosing suction then :

there is not enough water in the pool and the skimmer is sicking air.

bad pump seal

loose pump lid

bad o-ring seal at pump lid

loose fittings in and out of the pump. that is the piece of equip. that has the motor attached. The filter is the large tank. The pool is the vessel that is holding all the water.

You can also lose pressure if your skimmer and your pump baskets are filled with debris, kink in the hoses, or your filter needs to be cleaned, or filter sand or cartridge needs replaced.

You also may be thinking of pressure as in the force of the water coming out of the return jets. If this is the case, then your filter has become dirty and the water can't pass thru it very well. Clean your filter.