Check Your Spider Gasket (Thanks Poconos)

[aylad]

Sometimes sand blowing back into the pool is caused by an oversized pump for the filter, and sometimes it's caused by a crack in the laterals in the filter. Sometimes though, it's just dust/pollen/dirt/airborne junk that settles in the water and you can't really do anything about it but vacuum it up.

Janet

[Poconos]

Glad to hear you resolved the problem. Those blasted gaskets can cause all kinds of problems because when they go bad you can get all kinds of unwanted flow paths, including lost water out the waste line.
Al

[Big Mike]

This is very true, I also noticed water going out to the waste line the same time as the pool was filtering.