I'm guessing the problem is with losing prime, not with the filter. The filter is just where the gauge happens to be.

Check at a return, and compare flow when the pressure is high, with what it is when it "loses pressure". Low flow with low pressure USUALLY means low pump output.

A very common cause of prime-loss is a suction leak, often at the pump strainer lid gasket. If you want to send pictures of your pump, filter & piping, we might be able to spot some other places to look.