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Examine your filter grids for tears/holes/damage. Inspect the manifolds closely for cracks. Make sure any o-rings are in good condition and seated. Check the air relief valve strainer/screen for damage and make sure it's seated.
The bottom of the grids were likely submerged in water during the disassembly process which may explain why there is no DE there.
How are you testing your water?
Can you post a full set of test results?
An
Overnight Chlorine Loss Test will rule out algae.