It is said over & over again not to backwash a sand filter until the pressure is about 10 psi over the normal reading. With mine, at just 4-5 psi over normal, the water doesn't seem to be flowing into the skimmer very well & the water in the skimmer is barely moving & dust, dirt, etc. is floating on top of the water in the skimmer. Is that normal or should I go ahead & backwash?