The suggestion list returns only the members that are working on a specific project. Specifically, these users are collaborators (team managers, translators, reviewers, language coordinators) as well as project maintainers.
If you are trying to find an organization administrator user though, who hasn't been invited to the translation team or as a project maintainer, then this user won't appear as a suggestion. In such cases, there are the following 2 options:
Include the username in your comment even if this username is not one of the results the suggestion list returns. If the username is a valid one, the user will still get notified.
The administrator user can be assigned to an extra user role (the ones mentioned above) that are related to the specific project so that he/she will be considered one of the members that are responsible for the localization of the project.