Email notifications are especially hard to diagnose becuase they travel through many systems from the original sender to the final destination. Emails can fail or be blocked at any of those transfer points.
To troubleshoot and get to the bottom of it, I would first ask WF support to confirm the emails went out in the first place. If they didn't, check your users notification settings. If they did, Workfront cannot control that. I would create a test account with an email address from a more public email service; gmail, hotmail, Yahoo, etc. and see if it comes through there. If so, it could be your organization has inadvertently blocked it.
Then you'll have to go to your I.T. department and have them look at it.
