have you checked it is not sorted out by a spam filter or some rule?

if you end the failed workflow, do you get a notification?

Thanks for replying. The notification is not affected by a spam folder or rule - I am getting other notifications. And, I did get the workflow cancelled message. Upon further investigation, I think it had to do with a redundant action. The action before the "Start workflow" action was to change a status. There was another workflow that was set to start upon a status change. Even though Nintex saw it as an error because the Start workflow action didn't work, it didn't send a notification because the workflow it was supposed to kick off was already running. That's my hypothesis at the moment. Have tried removing the redundant action so hopefully that will fix the problem.

start workflow action has an option " Do not start the workflow if it is already running"  to prevent errors if a workflow is already running.