Debris and small amount of air coming through the return that the Polaris goes in

I am puzzled. We have a triton II sand filter 08ETR60. The pool is 13. I have replaced the sand one time. I realize that it may be time to replace it again, but think I need to find out why debris is returning into the pool through only one return....the return where the Polaris plugs in. I can run the pump for 8 hours and the little screen that is in the plug is full of debris. If the Polaris is out you can watch that return and see a small amount of air and debris coming back into the pool...only through that return. I didn't know if lateral damage could do this or if the piping for the Polaris might be the cause. Perhaps I didn't get enough sand back in the filter when I replaced it a couple of years ago. I read something about "freeboard" a measurement from the top of the sand to the top of the filter. I couldn't find specifications on that though. Any help or suggestions would be appreciated.