Why is the pressure on my sand filter low?

Clogged filter or if it is an older pool it could be a warn impeller (the thing inside the pump that moves the water. It could be clogged pipes from built up debris. If you remove the jest you can check for built up debris as It will generally collect behind the jets. another thing is a break in the line that would relieve the water pressure and thus less pressure at the jets. lastly your bypass valve having an internal leakage or not fully engaged.