Jeff Mc

Hello all I set my pool up this year as usual. it is a Intex above ground pool with a sand filter (ECO20110 ECO15110 model) As usual new proper pool sand. this year for some reason pool pressure stays high and I have low flow coming out. seems to be in the filter system as I have great flow coming off pump, when I backflush it pressure drops down to barely in green with great flow for about 20 min then pressure goes back up and flow drops. Yes the whole filter has been disassembled cleaned very well and reassembled. Anyone know what all can cause this issue please help