Wsas a rinse done after the backwash? If backwashing is not done long enough ie 30 seconds instead of 3 minutes, and if the rinse cycle is skipped or too short that can cause sand back into the pool. Each filter is different and the amount of time it must backwash varies with how dirty the filter is. The sight glass should be monitored. When the water goes from dirty to clear, it’s the indicator that it has been done long enough.