After the hand finishes, reset the game to the state where somebody misplayed, and then talk it through. What information did each of the players actually have? How did the player who needed something to happen try to signal that's what they needed to happen? How did the other players understand that action (or lack of action)? Was there a way the signaling could have been clearer? If there were multiple possibilities for what the signal meant, could the other players have disambiguated it somehow?

This should not be an acrimonious process. Think of it as a blameless postmortem. The goal is not to decide who made a mistake, it is to understand how to get better as a group and avoid losing the game the same way again. It will also help everyone empathize with the other players, since they'll actually understand what information the other players had and what their thought process was.

As you get better as a group, you should quickly get to a state where either it's clear to the player who misplayed that they did in fact make a mistake (and they'll apologize before the hand is even over) or everyone agrees it was unavoidable.