We just opened our pool. Everything was fine then we noticed the PSI was high. We checked the skimmer lines, the filter basket and propeller to make sure there wasn’t a blockage. We backwashed and cleaned the filter. The pressure came down but then went back up. We checked the valve, the gauge and even replaced the slats in our DE filter. We thought that solved the problem but a little while later the pressure rose again. Don’t know what else to do? What’s wrong?