You may have hidden an account in an audience when it wasn't relevant. This article demonstrates how to unhide an account.
1.1. The hidden accounts option appears under the filters dropdown. If you haven't saved a filter yet, create a test/dummy filter by clicking the "Save Filter" link.
1.2. Enter a name for the new filter and click Save.
2.1. Expand the Saved Filters dropdown and select Hidden Accounts.
2.2. Select the checkbox next to the account(s) that you want to unhide.
2.3 Click "Unhide Selected Accounts" in the action menu on the right-hand side.
2.4 Optionally delete the test/dummy filter by hovering over the filter in the dropdown, selecting edit, and Delete Filter in the pop-up.