I have a 12 X 27 onground/partially inground pool with a Hayward cartridge filter. Normally a fresh/clean filter starts at about 12.0-12.5 psi and I remove to clean at about 18.5-19.0. After I had my pool opened by my pool supplier crew I had to top up the water level before (a day later) starting it up myself. The discharge pressure started out at 17.5 psi. I was surprised by this since I bought a new filter last season and cleaned it well before storing it for the winter. In any case I decided to clean it with the special filter cleaner solution - immersed it overnight and cleaned out all the pleats with a hose. I didn't get much dirt out, since, as mentioned, I had cleaned it last fall at shutdown. I started up again and I have the same 17.5 psi reading; it's now 18.0 psi after circulating for 4 days. I have checked for air leaks on the suction side and any obvious obstructions or diversions on the discharge side but can't see anything. I hate to buy and try another new filter so soon, but I've run out of troubleshooting ideas.