A family of Microsoft products that enable users to capture, organize, and reuse notes electronically.
I had this problem too, turns out I have two accounts (personal and professional, they have different emails) that I was logged in in the same time, and a notebook on each, both failing to sync. Shared each notebook with myself (both of my email addresses), and both notebooks sync now.
Also, my notebooks were not set to sync automatically. Make sure that "sync automatically whenever there are changes" is chosen on the Shared Notebook Synchronization" page (opens if you right-click the tab for your notebook and chose "Notebook sync status")