Easy answer for AF sanctioned stakes. Revert to a 1,2,3. As long as the other judge agrees, of course. Honestly, I have never needed to do that, but it was almost necessary a couple of times.
The best performance of the trial may not be of championship caliber and therefore, should not be awarded a CH. However it is still the best performance and deserving of a first place placement, IMO.
In AKC stakes, it can be a little different because of their points system. If a performance is not worthy of championship points, the placement should, IMO, be withheld. This is actually done with some frequency by AKC judges.
I recognize that physical conditions on the ground sometimes are such that a top tier performance is impossible(excessive heat, sleet/snow rain, no bird contacts,etc.), and no fault of the dogs or handlers. You do the best you can with what you have to work with.
I know that many folks out there have vastly more experience training, judging and handling dogs, and opinions may differ...but like I said earlier, if a club asked me to judge, they agree to live by my decisions.