Not getting bell notifications to approver

Summary:

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Hi,

When I am submitting the requisition for approval, as an approver, I am not able to see any notifications in the bell notifications area. However, I am able to see the notifications when I go to Worklist area from there.

I have updated notification mode to all in bpm administration page. Also, I have ran synchronize notifications in the global header multiple times. It is only synchronizing past notifications. No new notifications are coming in. Just to update, this is also happening for PO, invoicing across the instance for all notifications. It was working well before a week.