Welcome to the forum. First you have to confirm that it is a DE filter. I never dealt with them but I do know that after you backwash you need to add more DE. In your case the fikter looks plugged in the filter direction. I think I remember that if you run them without DE powder then you run the risk of damaging the grids or membranes. Could be plugged in one direction with the crud you vacuumed. I'm making an educated guess on the previous comments. Someone else will know for sure and also suggest how to clean them. Would be helpful to have the pump info too. That will give an very rough indication of the max pressure you can hit with fully blocked flow. I'm wondering how close that 20 psi number is to the blocked pressure.
Hope this helps a little.
Al