I have a one year old, in ground pool and spa with a DE filter, using cellulose fiber. I cleaned my filter within the last two months. I noticed that the water features and spa spillway didn't seem to be flowing as well as they should. On checking, the filter pressure was up near 28-30 psi (the "dirty" range). When I back flushed the filter the pressure dropped to the clean range but returned to high pressure and poor flow after 45 minutes to an hour. I've repeated this a couple of times. I disassembled the filter to look for obstructions. I cleaned the filter and removed some wadded cellulose fiber which I thought would solve the problem. I assume there is an obstruction somewhere in the lines that is moving when I back flush. It may be wadded cellulose fiber somewhere but I'm not sure. I'm guessing the next step is to try to back flush the lines from the skimmers (there are 2) two the filter using a shop vac. Is there a way to do that? I assume that there is some sort of check valve that would be in the way. Any ideas would be greatly appreciated.