Our pool has had this issue all summer. After the filter is on for 3-4 hours the pressure jumps from 12 to 20 and very little comes out of the spa jets. We replaced the DE filter cartridges and all was fine for about 3 weeks. Then the pressure started jumping to 20 (Note: we did add algae killer right before this happened, but it I am not sure that caused this, probably just coincidence) , we did a backwash and all was fine for a day, then when the filters came on the next morning in 3 hours the pressure jumped again and very little was coming out of the spa jets. When I turn to vacuum only the pressure drops but at that point the vacuum starts going out of the water and sucking air. I can get the pressure to drop with even a quick backwash (about 5 seconds), but that will only last for a few hours before it does it again. The water level is fine. No visible cracks in anything, O ring on the filter is brand new. I am out of ideas ... any help is greatly appreciated!!