Activating mobile tickets in the Trainline app

In the Trainline app, when I've purchased a mobile ticket, I can download or view it, but can't find a way to activate it for use. There's a control that says "activate tickets" next to the view tickets button, but it doesn't seem to do anything. On a recent train journey where I was using mobile tickets, I had to let the train manager activate the ticket for me as they were doing the ticket inspection, because I couldn't find a way to do it myself. Can anyone explain to me how to activate tickets with VO? Thanks.