Extensions settings don't save?

I know I’ve run into this before but I can’t remember what caused it and instead of pulling my hair out any more I thought I’d just ask 😉

I’ve got an extension I’m building and associated extension settings. All the code looks like it’s set up correctly for the settings fields - all the fields are there and the proper language is being seen - but when I enter values and save them they just don’t actually save. If I go back to settings page everything is blank again.

What am I forgetting?

Thanks!