Rapid pressure build up in cartridge filter

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?

Algae.

Post ALL chem levels.

Live algae will clog a filter quick. I bet that's what you've got, just not enough for you to see the algae yet.

Test your water chemistry, preferably with a Taylor K-2006 or other FAS-DPD test kit. They can be purchased Post the test results here. If you have any algae starting to grow and chlorine in the water you will register combined chlorine on the test result. (Normally you would not expect to see any more than 0.5ppm CC) Here's a page on poolsolutions that tells you about he various test kits: http://www.poolsolutions.com/testkits.html

The solution will be to shock your pool until you register less than .5ppm CC and you also lose less than 1ppm FC between sundown and sunup (night because sunlight breaks down chlorine).

@Aaron -- FYI -- new members in the Getting Started section cannot follow links. (BTW -- Appreciate your help around here!)

@JohnB -- You can get to the chart that Kelemvor was trying to link for you that explains the levels needed to shock a pool through this link: http://pool9.net/cl-cya/ (But, you will need to copy that link and then paste it into a browser window after you first log out.)

I shocked the pool and maintained ~6-8 ppm FC with very little CC (per Leslie's test). FC does not change during the night. I frequently wash the filter cartridges but don't see anything coming off in the water besides a few small leaves. The pump flow is still very low even immediately after filter cleaning.

Is there a better cleaning method than a water rinse? Do I need to acid wash them?