What is the test for declaring a mistrial?

In my view, it does not appear from the record that any one of the three jurors could not have decided the case based on the evidence at trial. That is the ultimate test of the "manifest necessity" for a mistrial. Arizona v. Washington, 434 U.S. 497, 505-06, 98 S.Ct. 824, 830-31, 54 L.Ed.2d 717 (1978). By the time they began their deliberations the jurors would have been reminded of their duty to decide the case based solely on the evidence presented many times during the proceedings--first by the judge during jury selection, then by both counsel in the summations, and again by the judge in his charge. Why were those three jurors not reminded of this duty before they were dismissed? Before taking the extraordinary step of declaring a mistrial, it was the duty of the court to urge the jurors to continue to serve and to consider the case solely on the evidence presented at trial.