Sent Fax stuck in “Sending” status

Reasons a sent fax could be stuck in Sending status:

1. Authorization – The admin user may need to re-authorize the app under Fax Configuration tab. There are several reasons as to why you would need to refresh authorization, for example when there is a change in authorized user, anything in your faxing process/Salesforce has changed, an org refresh, or updated package install. After you do this step, Sent Fax records will either start to be sent within 24 hour period or you may need to recreate the sent fax records after refreshing authorization.
   1. If you see an error when you click the Grant Access button, copy the error and search the knowledge base for an existing troubleshooting article or take a screen shot of the error and email it to support@opero.com to open a case with our support team.
2. Permission Issue – If a fax is getting stuck in Sending status for no apparent reason, it could be a permission issue.
   1. Files/Documents Permission Issue – See this article for steps to resolve.
   1. User Permission Issue:
      1. Non-admin users must have a Salesforce Standard profile type (or equivalent permissions), the Fax User permission set assigned, plus the manually-granted system permissions listed here Fax User Permission Set.
      2. The Admin user who authorized the app must have a Salesforce System Administrator profile type (or equivalent permissions), the Fax Admin permission set assigned, plus the manually-granted system permissions listed here Fax Admin Permission Set.
      3. If a non-admin user is faxing files they do not own, they must have the permissions listed here: User Permissions List.
3. Automation/Inactive User –If you have configured an automatic process for faxing (using flow, apex, or any other means of automation), and sent faxes are stuck in Sending status, you should check if the running user is active. It is possible that one user may be set as a default user for the specific automation, and it got deactivated. Also, verify the running user has proper permissions (see Permission Issue above).
4. Sent Fax is Missing a Subject – A subject is required for any Sent Fax record where “Disable Cover Sheet Feature” = false. When a user attempts to send a fax without a subject from the Send Fax page, the system prevents sending and displays a “Subject Required” error. However, with fax automation, if a sent fax record is missing a subject, the sent fax record will be stuck in “Sending” status.
5. Attempting to Fax Encrypted Files – The library that our app uses to combine PDFs does not support encrypted PDF files. To prevent faxes being stuck in sending, users should not attach encrypted PDFs when sending faxes. 
6. API Call Limit – You may have hit your daily API call limit. Go to Setup > Company Settings > Company Information > Show API Requests > Last 24 Hours. See what the limit is and see if you went past it. If you did, you will need to wait till your 24 hour mark is over to start with new limit. You can submit case to Salesforce to increase your daily API call limit if needed.