20C - Vacation approvals not generating an email notification for one specific user.

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We have one employee who's vacation requests don't generate an email notification to his line manager. His manager receives the notification in his BPM worklist and can approve from there but often doesn't notice it until it's been there a while and in some cases the absence has already happened before he's been able to approve it. this issue doesn't affect any of his other direct reports. Any suggestions as to where to look to fix the problem?

Thanks in advance,

Mike