If your email alerts are not working for changes made to a document in SharePoint, the potential causes could be:

1. The user or group does not have sufficient permission to the document/list/library/site.
2. The user has not enabled alert notifications in their account settings.
3. The SharePoint environment is configured to not send email alerts.
4. The Email alerts are failing due to incorrect configuration of the SMTP server.
5. The email address set for the user is invalid.