Workflow status monitor show a notification sent to user but user is not getting the notification in

I have a custom workflow that shows the notification going to the performer in the status monitor but the user is not seeing this notification in their worklist.  I've tried to rewind the workflow and send another notificaiton out but the user is still not seeing this particular workflow in their worklist.  They have report seeing other notification including other custom workflows.  Can someone tell me how I can find the issue?  Thank you.