There are numerous possibilities. My thought is that early in the investigation the bag was probably partially buried and in dense brush as well as varying amounts of water. As months went by small and perhaps large animals like rodents and dogs pulled the bag closer to the surface and scattered the remains. What I'm getting is that early reports of the body being complete were wrong. Water may have also repeatedly risen and drained in the area, further agitating the bag and contents and scattering remains. Looking at pictures of the area it is obvious that it is dense and brushy with a lot of green vegetation. It would be easy to miss something in the area.
As far as cadaver dogs, AFAIK we don't have any confirmation that they were actually in that exact area, and if there was a lot of water when they were, they may have missed it especially if the remains were already mostly decomposed by then.