I'm having trouble with my sand filter spitting sand back into the pool. It is a 250 lb waterway filter I believe. Got it new last year and it spit sand back into the pool. Dumped the sand last fall as I figured I would need to replace laterals. Pool store gave me new laterals this spring and we looked over the riser and lateral body and could not find anything that looked wrong. Installed them, filled it up with sand, and now there is sand below each return again in the pool. Any ideas of how the sand is getting into the pool? I'm runing with a 1hp pump.