No alerts when content is updated or comments are added

Be sure that your users are aware of the following:
 

Content Alert emails are sent from the following address: content@salesforce.com. Have the impacted user check their inbox, junk mail box, and other filtered folders for this address to verify that the email was not overlooked.

Consider adding a allowlist for content@salesforce.com to the junk mail monitor for your organization or the individual reporting the issue.

Be aware that users with "Receive Salesforce CRM Content emails as daily digest" will only receive one email a day with any updates relevant to the user. This sometimes causes users to miss the updates as they are not sent with as much frequency as the live updates.
 

Users that report issues receiving Content Alerts should verify that they understand how the feature is triggered.
 

If you subscribe to a file, you are notified when a viewer adds comments to the file or when a new version of the file is published.

If you subscribe to a tag, you are notified of any newly published content that is associated to the subscribed tag (not when a new version is added to an existing document).

If you subscribe to an author, you are notified when the author publishes new content (not when a new version is added to an existing document).

If you subscribe to a Library, you are notified when new content is added to the workspace, including existing content that has been newly linked to your subscribed workspace (not when a new version is added to an existing document).
 

If you are still worried that your users are not receiving Alerts for content perform these simple test for verification-

Step 1. Edit the user record to meet the following:
 

Check "Receive Salesforce CRM Content email alerts"
Uncheck "Receive Salesforce CRM Content emails as daily digest" so that email updates are sent upon live edits.

Step 2. Isolate 2 documents that the user is subscribed to and do the following tests:

Add a new version to one document.
Add a comment to the other document.

Step 3. Isolate a Library that the user is subscribed to and do the following test:
 

Add a new document to the workspace.

Step 4. Have the user check their email looking for the Content Alert (be sure to have them check their junk mail and other filtered folders as well) and run an Email Log for the dates when alerts should have been sent.
 

Email Log files will capture Content Alert emails and the delivery status. To run an email log, go to: 

Classic: Setup | Administration Setup | Monitoring | Email Log files
Lightning: Setup | Environments | Logs| Email Log files

Search the log file for the email address of the user who hasn't received content alerts,in the log files there should be two entries for the user and each content alert sent out for the user-- as the first send goes to the internal mail server--and the second is sent out to the user's mail server.
 

See the links below for more information on the following:
Requesting an Email Log
Viewing Email Logs