It sounds like some of the pool water is bypassing the grids, or at least the DE on the grids. Open the filter and check for any split, cracked or missing parts, and ALSO for any part of the grids that are broken or not coated with DE. AND check for tears or splits in the membrane.

I'd focus on the upper parts, since I suspect you'd be more likely to see dirty water if the bypass were lower down.

Pay particular attention to the upper manifold (#14).

CAUTION: if you don't relieve air pressure before opening, the tank can 'explode' and hurt you!