I have a sand dollar sand filter and it does not seem to be filtering properly. It is two years old and I have never done anything to it other than the normal backwash and rinse. I vacuumed the pool the other day and it was perfect, I went back out the very next day and all of the sand I vacuumed out seemed like it was back on the floor. It may not be all of it but most of it is. I saw in another post where it may be channeled and to hit the side of the filter with a rubber mallet but when I tried that dirt started coming out of the return. Any ideas what may be happening and what I should do? Thanks for your help.