Lots of DE returning to pool - What am I missing?

Last year before closing the pool, it was operating normally. I opened the pool this year and found that much of the DE I am using is returning to the pool. Appears to be about 50% ends up in the pool. The filter is a Hayward DE 4820.

I have cleaned and inspected the filter grids, manifold and o rings. I don't see any tears or cracks. Internally the grids appear clean (no debris or DE).

I have inspected the multi-port valve. There are no noticeable cracks or other damages. The spring and washers look okay. I have replaced the multi-port o-ring as a safeguard.

I get good pressure in the filter.

The filter grids are 4 years old and may need to be replaced; however without finding any obvious cracks or tears I don't see how so much DE is escaping. So before I buy all new grids (and pray it fixes the problem) is there anything else I should consider?