I configured the Slack Notification in the global settings and in the job.  When in the global settings, selecting "Test Connection" successfully sends a notification to the Slack team.  When I configure the job, selecting "Test Connection" successfully sends a notification to the Slack team.

However, when the jobs is triggered, a Slack Notification is not sent.

I don't see any errors in the jenkins.out.log file and I don't know how to further troubleshoot. 

EDIT:

After troubleshooting, it turns out the the notification is NOT sent to the slack channel if the post-build action is in a "Flexible Publish".  So it appears the "Flexible Publish" plugin is interfering with the Slack notification from being posted to the slack channel.