Three weeks is a long time to get it cleared up, but remember that it took a long, long time for the pool to get in the shape it was in, so it's going to be a long process to clean it up. If you're still seeing improvement in water clarity, then the filter is at least working. Do you see anything blowing back through the returns? One thing you can do is add the handfull of DE into the skimmer and then watch to see if it's coming back through the return--if so, then the filter has a problem. If not,then it's trapping the particles and you just need to apply another dose of POPP--Pool owner patience and persistence! YOu might also open the filter up and just make sure that it has an adequate amount of sand in it. Your owner's manual will tell you how full it should be, but I think with sand the general rule is about 2/3 full.

Keep an eye on your pressure, and continue to backwash it when your pressure gets 8-10 psi over your "clean" pressure. I wouldn't backwash it any more often than that, because a dirty filter actually cleans better than a clean one.