Adobe Animate - Onion Skin does't work with past ffames

Hi!

I'm trying to solve this problem for a while.

When I activate onion skin, I can't see the previous frame. I'm sure I'm using onion skin correctly because I watched a lot of tutorials to be sure I was doing it well.

I just can't see the previous frame, I've tried a lot of solutions but... Nothing. Can you help me please?