First, please check these things:
- The folder with the project number is directly under the Inbox
- The exact project number is used
- You have recently dropped an email into that folder
If those things are correct, check that the "Categories" column is shown for the folder, and that the Filed in TonicDM category has a color other than white (see below).
Do emails file when you click the File Selected Email button in the Outlook sidebar? If not, please reconnect TonicDM and Office 365.