I had a similar problem - but it was on a new pool installation. I would backflush the DE filter to drop pressure (mine runs about 20 psi clean/new) every two days or so only to have the pressure creep back up again a few days later. I finally disassembled the filter and discovered that half of the filter was packed with DE - solid like clay. This was after I backwashed. Appearently the backwashing was not removing enough DE, and on top of that, I was adding 80% more (refill) DE powder each time. After I disassembled and cleaned everything out and added a 100% fill, the pressure has remained at the new (clean filter) level. I have not head to backwash again since. Its been about 2 months now. I bet I won't need to backwash all summer.
I think the new pool startup loaded the filter with plaster dust that would'nt flush out. I made the problem worse by adding more DE after each backwash than what was coming out.
This example may not be of much help other than to tell you that you may need to disassemble. flush/clean out, and start over to correct the problem. You new filter may have pulled a lot of dirt/pollen out of the water that your old filter didn't catch and now its clogged (like mine was with the plaster).
Good Luck.
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