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 drop-down. 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 Filter drop-down 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 Actions menu on the right-hand side.
2.4 Optionally delete the test/dummy filter by hovering over the filter in the drop-down, selecting Edit, and Delete Filter in the pop-up.