I was going to start cleaning my pool to get it ready. It is green. No chemical in it yet. I have ran filter all winter at least 4 hrs per night. Today I put my creepy vac on to start working on the bottom. I notice the pressure was over 40psi, so I figured I better backwash and rinse. When I first did backwash noticed some sand came out I let it backwash for several minutes and then rinsed for several minutes. I let it vacuum for about 30 minutes and checked the pressure was back over 40 psi so I again backwashed but after several minute no water was coming out the waste line. So I rinsed again and then put the dial back to filter and let it run again pressure went right back to 40. so I tried backwashing again it bring the pressure down to 30 but nothing lower. But pressure goes right back up. I change the sand last year. I have very strong water return to the pool. Any suggestion as what might be going on?