So I backwashed for the first time this season and noticed the pressure isn't going down. When the pool was first put in, the pressure was 12 psi in 2008. I would wait until the pressure would build up to about 16, then backwash (and rinse). and it would go down to around 13. This year, the pressure started out around 18 psi, and when I backwashed (and rinsed), the pressure stayed the same. The instruction manual says that if you backwash and the pressure doesn't go down 4-6 psi, then you should replace sand. It also says that you should backwash when the pressure gets up to 10 psi more than the starting pressure, which it hasn't yet. What should I do?