Hi Irith,
Not sure which link button you mean.
The one that is set in the "link to next activity" in Lesson settings? This link appears on the "end of lesson page" which usually shows the student their score before returning to the course home page.
Sometimes showing and not showing has to do with the browser cache. It is usually a good idea to refresh the browser cache so Moodle can tell the browser of the new changes to the page. This happens to me all the time. Probably not your issue.
Chris