The body-cam video suddenly turns shaky as an Ohio police officer fires his gun into a car, leaving an unarmed black motorist dead, shot in the head. In Wisconsin, slow-motion images show an armed man lobbing his gun over a fence, then falling backward, before a police officer fires a fatal shot into his chest. And in Minnesota, a police dashboard camera captures an officer abruptly firing seven shots at the driver of a car he has stopped, as the man’s girlfriend and her young daughter watch from inside the car.

Each of these videos held an essential role in a courtroom this month, as jurors tried to decide whether the officers had committed crimes. Yet none of the cases brought convictions; in Cincinnati, the trial of the Ohio police officer ended in a mistrial on Friday, the second time the case had been tried without jurors’ agreeing on a verdict.

Since the 2014 shooting death of Michael Brown by a police officer in Ferguson, Mo. — a shooting that was not captured on video — there has been a nationwide push for more video recording of the police. But, as the three cases this month show, videos by no means ensure convictions.

“It’s not the end-all, be-all,” said John T. Chisholm, the district attorney for Milwaukee County, who presented video from two officers’ body cameras in the case against Dominique Heaggan-Brown, a police officer who fatally shot Sylville K. Smith in August. On Wednesday, Mr. Heaggan-Brown was acquitted.