Dirty return water when vacuuming

Aug 12, 2009
3
gresta46001 here. New to this forum, and obviously have a problem with my pool.
21,000 gal, IG Vinyl, DE Filter, 1HP 11/2 inch pump, Hayward filter (size unknown).
The filter has a Hayward Vari Flow valve on it.

When I am vacuuming the pool, it works well for about 5 or 10 minutes, then the water coming from the return starts moving the dirt back into the pool. It appears the filter is not functioning properly, but is filtering most of the dirt out.