User isn't getting Bell Notifications upon setting up the Rule

Summary:

Upon setting a FYI approval notifications for user, unable to receive bell notifications. In this case, I am trying to get a FYI notification when a Dropship-PO is created for supplier. I created a drop-ship sales order and processed to requisition and PO, still I did not receive any notification.
I checked the FSM rule and also BPM=>Notifications Mode=> ‘In-app mode’ (This is enabled) no issues here.
If you create a Drop-Ship SO and process to Req=>PO upon clicking ‘Manage Approvals’ =>Rule is successfully triggering but unable to receive notifications..
kindly let us know what would be the cause for this issue