I think I finally figured out how to maintain my DE filter. Maybe this will help others. First of all I bought a "one pound" DE cup at my pool store. Problem is that it only holds a half pound. So I was adding only 50% of the required DE.
Second problem was backwashing the filter. I used to backflush until the water came clear. Then I accidentilly discovered that I need to BF for a few minutes, turn the pump off, BF a few more mintues, turn the pump off, etc until clear water comes out.
So now a couple weeks before I close the pool. I have crystal clear water for the first time in a long time.