A few common reasons: accounts closed or terminated on Meta aren't monitored; accounts that have never spent and have no payment problem are left off the list to avoid clutter; the With delivery filter (on by default) hides accounts with no recent spend, so try turning it off; Problems only mode shows only accounts with problems; and the account may have been hidden, so check the Hidden button. Also, if you use an account selection in Ads Editor, the Monitor only shows the selected accounts. Finally, check that you're on the right tab (Meta Ads or Google Ads).