From Longman Dictionary of Contemporary Englishsomebody’s mind is not on somethingsomebody’s mind is not on somethingATTENTIONif your mind is not on what you are doing, you are not thinking much about it because you are thinking or worrying about something else His mind didn’t seem to be on the game at all. → mind
Examples from the Corpus
somebody’s mind is not on something• I was trying to study, but my mind just wasn't on it. 