I'm new to Sand Filters. Just got a new 350 lb one, which has been in service 1 week cleaning a very dirty pool. The first time I turned it on the PSI was around 9, give or take 1. The first couple of times I backwashed, it brought the pressure back down to 10 psi. But now when I backwash, it only drops to 12 psi when it's done. My procedure has been backwash for 2 minutes, then rinse for 30 seconds. Should I repeat the procedure if it doesn't bring it down to 10?