+ Your sand may be gone, typically because the filter is too small for the pump, and you've blown it out during backwash.
+ You may have coarse sandblast sand in your filter, instead of properly sized filter sand.
+ Your pump may be much too big for the filter, and is blowing the dirt through sand -- if your CLEAN filter pressure is over 15 - 18 psi, this is likely.
+ Your sand may be clumped from sun tan lotion, Baquacil or other contaminants, allowing bypass channels to form.
+ Or all of the above could be true.

Since your builder (I assume) sold you a Nature2 unit, the 'bogus-ness' factor of your equipment set up is likely to be high.