Make sure that the Monster you own is hatched and placed in a habitat.

Also, remember that some collections have more bookmarks. (On a left side of a book)
Basic and ranked:
  • The basic collection requires all the Monsters to be hatched.
  • Ranked up collections of the same Monsters requires having these Monsters ranked up

So remember, if you see that some of the portraits are grayed out TAP on the bookmark with a star symbol on the left to switch between basic and ranked up collections.

As you see below, I own a basic Tyrannoking.

If I tap on the Bookmark with the star (Ranked 1 Monster Required) my Tyrannokyng is grayed out.