Outlook may have disabled the plugin.
Step 1: Go to File -> Options
Step 2: Choose Add-ins -> Manage add-ins.
Step 3: Press the “Go” button at the bottom of the screen. This will take you to a new window where you can see which Add-ins are active. Ensure that the MeetingBooster add-in is ticked and then press OK.
This will re-activate the plug-in and ensure that it stays active whenever you start Outlook up.