Starting from scratch won't really help, if there is no evidence there is no evidence, and finding any more after three years is a tall order. The real problem really is that the initial investigation was poorly done, and that has pretty much contaminated everything else.
In a murder investigation the first 48 hours are crucial when it comes to evidence gathering. After that things fall off rapidly, and in particular, the longer it takes to become aware of potential evidence, they more likely it will be that the evidence is gone or have become contaminated. After three years the chances of any physical evidence will be extremely low.
What is particularly noteworthy is that since finding the body, no search warrants have been executed that we know about. What that means is that whatever was gathered in the first few weeks is now no longer considered probable cause. Chances are that subsequent investigation refuted most of what was suggested in the initial affidavits, and that is the real reason why no progress is being made now. They know she is dead, but they have nothing to follow up on.
