Since I opened my pool in mid May my pressure has been at 9 PSI. The past few weeks has slowly raised to 13 psi. Yesterday I returned from vacation and ran my polaris. The pressure climbed from 13 to 20 PSI. I backwashed and replaced the DE, still as 20 PSI. I even replaced the gauge. I do have less flow out of the returns than I have had. I ran the filter in recirc and have improved flow. There has to be someting going on with the filter. What should I do?