Facebook manipulates the feed to show only those posts which guarantee maximum user engagement. When a new post is made, the post is initially shown only to a small subset of fans. It will be shown to more people only when they engage with your post or if you cough up money to promote the post.

Here's science journalist Derek Muller (of Veritasium) describing his observations on this issue in detail: https://www.youtube.com/watch?v=l9ZqXlHl65g