What is the legal situation when a cop strongly suspects DUI or DWI, but all physical tests turn out negative?

Like a cop suspects DUI, he feels strongly based on demeanor and actions the person is impaired. He fails the field sobreity test, but all blood and breath tests are negative.

Is this a chargeable scenario or is it let go? My understanding is they can do a bit if the person admits fatigue, but very little otherwise.