Cannot see all the boards within a team

@Alessia Butler - This is also outlined in the Admin FAQ Help Center article:

7. I only see 5 boards, but in the team settings, it says that there are 8 of them. Why can't I, as an Admin, see and manage the rest of the boards?
   - If you don't see some boards it means they have a private status and only their owners can see them. You can reach out to your team members and suggest they share the boards with you.
   On Enterprise plan, it is possible to enable Content Admin permissions to manage all boards and projects in the account.

Source → https://help.miro.com/hc/en-us/articles/360034270533-Admin-FAQ

In short, the boards are set to “private” - more on this here → Public boards vs. private boards.

However, if you were specifically asking why as an Admin cannot I not see/administer these boards, this is the way Miro has defined the Admin role at this time. Looking at the Admin FAQ, it does look like they have added a feature for Enterprise Plan Company Admins to administer private content, or at least that is the way I am reading it - more on this in the Content Admin Permissions article.

Wish List Ideas

There are  several Wish List Idea posts where other users are asking for additional features for the admin role -  you may want to upvote some of them: https://community.miro.com/search?q=admin&content_type%5B0%5D=ideation