Notification bell

Notification bell

our notification bell has stopped displaying alerts for "requester replies".
 
AS far as I'm aware, nothing has changed on the system side of things other than a reboot.  Other notifications such as ticket assignments etc are being notified via the bell - it seems that only requester replies have stopped notifying.
 
Is there a method/command to switch these back on ?
 
many thanks
ewan

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