Contacts are in the queue. Most of the time, this is because contacts have set how often they want to receive emails from you on the email settings page. If a contact wants to receive only one email per month, the second email remains in the queue.
If a contact has this set, it looks like this:
If you don’t want to provide this option, you can disable it in the email settings. At the bottom you will find the logout settings: “Show contact frequency preferences”.
How to create a Preference Center/Update Settings page for your emails and newsletters.