Why I cannot add some of the Social Science courses found in "explore" to dashboard?

I am a college student in HK. I am trying to take some humanities courses on this site to improve my academic vocabulary. But I realized I couldn't add some of the courses I found in "Explore" to my Dashboard. For example, AP®︎/College World History, I can find it in "Explore", however, I cannot find it in the course list shown after clicking the "Edit Courses" button at the top right. Meanwhile, when I was in high school I took AP®︎/College US History on the site; at that time, it could be found in the course list shown, now it disappeared from the list and can only be found in "Explore." Could anyone tell me what's going on? It's really inconvenient that I have to find the course in "Explore" every time when using the site. And why would the course disappear? Is that some kind of site update?