One Comment Added notification was not received while others were fine

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We are using Jira Cloud.  I am having intermittent notification issue when a comment is added to Jira Service Desk request.  Sometimes reporter did not receive comment added notification. 

I tried three requests and added comments to all three of the requests. Same agent and same reporter.  Reporter did not receive one of the notification while being notified of all other comment added.  Then agent added the comment to the same request again.  Notification worked this time.

I have used Notification Helper.  It all looks good.

Please help point me to something I may miss checked.