Incomplete List of Monsters/NPCs when trying to assign/link to custom tokens

I have made many of my own custom tokens for use in my games. When I drag and drop onto a page and then go to the drop down menu "Represents Character" to link the token to one of my purchased books on Roll20, I only get the monsters/NPCs from the Monster Manual and not the monsters/NPCs from my other books, e.g. Volo's Guide to Monsters, etc. For example, I have made custom NPC token for Archers, but the drop down menu won't give me the option to select the Archer stats (just not there at all) from my purchased Volo book. What's up here? Am I missing something, doing something wrong? Any advice would be appreciated. Thank you!

You can't alter what is in the compendium.  If you have dragged out an archer so that a copy is in your journal, you should be able to assign a new token then.

@Kraynic thank you very much, that did the trick! I knew there must be something I was doing wrong. Much appreciated.