It depends on whether the umpire judged the fielder did not make the catch or the umpire was out of position or blocked out and could not (did not) see the catch/no catch. Or, with more inexperienced umpires, it could be a question as to who was responsible for ruling on the catch. That is one of the things that the umpires should discuss in their pre-game conference. Each umpire has his own responsibilities on the field and one of the umpires, but not both, was responsible for ruling on the catch/no catch on the play. Who was responsible depends on such things as the umpires' position on the field, the location of the hit, and perhaps several other factors. The responsible umpire may go to his partner for help, but it could be that his partner was watching some other aspect of the play and could not provide any additional information. Or his partner may have seen something to help the umpire make the call. Regardless, the final call rests with the responsible umpire. One umpire cannot overrule another on a given play. If the umpire simply did not see the play, the accepted practice is "...if you don't see the out, you can't call the out..."

See an out, call an out!!! Never call an out without seeing it.