My pool is 17000 gallons in-ground with a Hayward 1 hp pump and Hayward 2420 DE filter and I can't seem to get any flow when in filter mode , I backwash and it's good for 30 seconds but then back to 18 psi and low flow. My levels and numbers are good cl 6.0 in shock mode, ph 7.5, alk 110 ppm. I am trying to open but can't get it to filter and stay within 5 to 14 psi range that it stayed in for the last 4 years. Please help. By the way , this is the ultimate place to come to for help, so many knowledgeble people and always with the best solutions.