To keep your pool water clean, the filtration system must be in good working order. If you notice low water pressure in the return pipe, first check the water level in the pool.
The water level should be at around 3/4 up the skimmer door's opening. If it's too low, the pump will have trouble priming and the water pressure will drop. The pressure gauge on top of the filter is also a good indicator of where the problem lies.
If the gauge indicates lower than normal pressure, there might be an obstruction or flow problem before the pump, or maybe even a leak. A high pressure reading means your filter system isn't allowing water to flow through fast enough. Do a backwash to flush out contaminants in the filter. You might also need to clean or replace the filter cartridge.
Explanations for your normal pressure can be found in the video.
