If your Office365 account has not been authorized for sometime you may need to manually sync for missing emails.
This is how you do it:
Click your name located on the top right of your screen and select O365 Authorisation from the dropdown list.
You will see the following popup display.
Click the blue 'Manual Sync' button.
You will see the following popup display:
Select the time period from/to and the mail folder that you want to search and sync missing emails from.
Press the 'Search' button first and you will see the results.
Once you have done this and are happy with results press the 'Sync' button.
This may takes some time depending on the number of missing emails.