I realised I'd left a key combo off, so I uninstalled the keyboard layout, made the change, and reinstalled it.
I now cannot have any custom keyboard layout. No matter which keyboard layouts I have active, I just get stock US QWERTY. I was previously running a modified version of US-International QWERTY - that has the dead keys for making umlauts and such but I added to it the capacity for a number of other foreign characters and symbols. It's the only keyboard layout I have active right now, and yet not even the dead keys are working.
My attempt today was to install a Dvorak layout with the same mods. Installed it, it was working fine but wouldn't show up in the language bar so I couldn't switch between Dvorak and QWERTY particularly easily. But I went to type an A with the circle over it, and it wouldn't work...turns out I'd forgotten that one. So I uninstalled my modified Dvorak keyboard, went back into Keyboard Layout Creator, tweaked it, and installed it again.
I'm now stuck on stock US QWERTY no matter what I do. I have two of my modified Dvorak layouts available to add, for some reason, despite having uninstalled it, and neither of them is the corrected version (I gave it a different name just to be sure, and it just doesn't show up on the list to choose from even though I've installed it).
I don't get it.