The possible reasons for Operators Not Receiving Workflow Notifications Are :
1. The Operator is not added to the GROUP which is selected in Campaign Properties -> Under General Tab -> In Supervisor(s).
2. The Email Address of the operator is Quarantined Or Blacklisted.
3. The Template is not selected in the Workflow Properties -> Execution -> Error Management.
4. And, last thing for the Application Server 6.1.1 build 8797, Adobe Campaign has some bug, due to which if some workflow is failing multiple times very frequently, then the notifications are being skipped by AC.
For Ex: If workflow is failed 5 times, you may not receive the notifications all 5 times.