I replaced the 4 filters in my Hayward cartridge filter in August. In about 3 weeks the filter pressure increased 10 psi (from 4 to 14 psig) so I cleaned them. Since then, the time needed to gain 10 psi is progressively becoming less and less. Currently, the pressure starts at 10 psig after cleaning then increases to 30 psig in just a few hours! I'm not seeing much, if any, dirt or algae on the pleats, but removing the cartridges then replacing them always results in the pressure dropping back to 10 psig then rising quickly to 30 psig.

I've run the pump without cartridges and get strong flow (filter pressure around 4 psig) so it appears the blockage is in the cartridges themselves.

Any ideas?