DE in bottom of the pool

4 year old Pentair FNS Plus FNSP-36 Vertical Grid D.E. Filter with Pentair multiport. Using a Pentair Superflo VS Variable speed pump and having the following issue:

DE in the bottom of the pool a few hours after backwashing and vacuuming. All grids taken apart carefully and inspected, top and bottom plastic grid housings inspected for cracks. No cracks. As a precaution replaced Air Bleed assembly, Replaced O-Rings.Talked with local Pentair pool dealer who said increase pump speed above 2000 RPM on low end as the DE maybe falling away from the grid. After about 1 hour after backwashing and cleaning the pool. A small cloud of DE starts to come out of returns in pool and steps.

Interested in anyone having any suggestions in helping to eliminate our issues!

Many Thanks