You can review the troubleshooting information on searching by tags to view the correct results. If a search yields no results, the reason might be how the tags are being treated and which tags are applied to the emails.
Filtering emails by tags for reports that are in Reports-New or for a list that is in Advanced Search results in no results if ALL tags that are being used are not on the individual emails. Look at the relationship as a many to one (many tags to one email).
A comma between tags acts as an AND not an OR. Therefore, all tags need to be on all emails that are being sent.