Strainer choking mechanism in pumps?

we have 9 stage centrifugal pumps installed at the pumping station of 18'' dia petroleum product pipeline. We have Y-type strainers 40 mesh (i.e. 40 holes per inch) installed at the pump suction. These are found to give different values of differential pressure across it ranging from 0.5bar to 2.5bar. The new strainers give 0.5bar while old ones distorted but rectified later are not giving any better perfomance. Please elaborate the contributing factors affecting the Differential Pressure of strainers.