No sub-folders in Outlook (Office 365 version)
If you experience the issue where you click on the arrow net to a main folder and the sub folders don't populate, I fixed it recently like this:
First close outlook.
Windows search, and then type in Run (when working remotely), or if at the PC, just press the Windows Key + R.
Enter the command outlook.exe /resetnavpane
Press enter, then restart Outlook and you should find your sub folders and their contacts display correctly.