I keep getting a little bit of sand in the bottom corner of the pool. I vacuum it up and with no one swimming a few days later it is back. Usually a line about 1/2" wide bu about 18" long. Any ideas where this may come from? Any how to fix it? It has done it from day one and we have had the pool six years. I'very just put up with it but the wife keeps asking to get it "fixed".