It used to open automatically on my second screen, but I must have accidentally done something to change that. When using 'play in window' the 'X' option doesn't work. How do I get it to change back to how it was?
MacBook Pro 16″, macOS 15.6
When playing the slideshow on a two screen setup, the letter X is supposed to switch the contents of both displays. I gather that is not working for you.
What is "play in window"? When clicking the Play button ▶️, the slideshow displays in full screen for me (and that is what I would expect).
Perhaps you need to make some adjustments in Keynote->Settings, under "Slideshow".
Trying to force Keynote 'play in window' to second screen
