Expense approval notification emails not getting sent to approvers.

We are having intermittent issues with approval emails for expense transactions not being sent to the approver's inbox. There does not seem to be any common denominator to the types of expense transactions or users that submit them. One happened on a regular expense report, another happened on a PPM expenditure batch. We've had our O365 team pull email logs, which show where transmissions between Oracle and Outlook are happening, but the users do not have anything in their inbox. Junk folder and settings have also been checked.

Attached are the transmission logs showing the emails sent to approver and the search logs showing no results in the user's inbox.