I'll let Facebook explain how Top Stories works:
When you sort by top stories, we'll try to show the most relevant news since you last checked Facebook.
We determine whether something is a top story based on lots of factors, including your relationship to the person who posted the story, how many comments and likes it got, what type of story it is, etc. For example, a friend’s status update that isn't normally a top story may become one after your other friends comment on it.
This means that a post with many likes, shares, or comments, is likely to dominate the conversation. It may be from fifteen minutes ago, but it is probably much, much older.
My personal recommendation is to toggle your view to Most Recent. This will make it easier to make sure that you don't miss any updates. This is VERY easy to do.
Click SORT at the top of your newsfeed. A box will drop down:
Click MOST RECENT. The posts on your feed should reset. You can always return to Top Stories if that's what you'd prefer, or you want to know what is popular.
What's a top story?, Facebook