The filter is a Purex Triton FNS Plus 48 for an in-ground pool.
Over the last couple of months a layer of DE has begun to cover the pool bottom. When vacuuming, the DE just pours back into the pool from the returns. Adding fresh DE after a backwash results in the same. I inspected the filter grids and they do not appear to have any obvious tears (though they are 12+ years old and appear to be installed properly. I first noticed this last year after a filter cleaning though it only allowed a small portion of the DE to enter the pool then. Now it acts as if there are no filter grids...
To me, the likeliest thing looks like the filter grids even though I cannot see any obvious damage. Can anyone think of any other possibilities I should be considering?