I find with ITV hub that I can only select subtitles when the programme has started, not when any of the adverts are playing. I only access it on my laptop but I have to wait until the programme itself starts, move the cursor so the options (play etc...) appear at the bottom of my screen, then click on the subtitles.
I do remember there was one of the online catch ups that I used that you had to be really quick at the beginning of the programme to get the options for subtitles, or it disappeared. I can't remember which one it was though.