It might be because your following feed is just a list of all the posts from people you follow, organized by what is most recent. Popular posts are not given priority, so if you don't happen to be looking when they are posted, you can miss them.
...it can seem more random than that because it also brings up replies by people you follow (depending on your settings) and re-posts. So if something is popular enough, people you follow may be interacting with it, and it'll come up more often.
Comments
I am following you quite a while but your posts aren't in my timeline. Saw it on Insta. very strange.