Hello Grimoire-
Generally speaking, you’ll need two things for your post excerpts to work. It sounds like you’ve already got the settings configured, so the second thing you need is the Read More Tag in your actual post content.
Edit your post, and set your cursor at the spot you’d like your excerpt to end, and click the Insert Read More Tag button (it looks like this: ).
Then, only the text before the read more tag will show in your excerpt.