I took my 12 speed cassette off a couple of weeks ago. I put it back on and didn’t get to ride it for nearly 2 weeks. Riding it yesterday it wouldn’t change into gears 9 and 10. what the hell?
It turns out there are 2 spacers in the 12 speed cassettes and they are different widths.
Removed cassette, found the spacers and put them in the correct order, and it now works. Whose idea was it to have different width spacers? Far out.