Right-click the notebook name in the Notebooks list, click Sync, and then click Sync This Notebook. Here’s how to manually force notebook synchronization in the different versions of OneNote:

Many unexpected sync problems can usually be resolved by forcing a sync as soon the network is available again. OneNote will automatically store pending changes to your notebooks whenever you are offline and then sync those changes as soon as a network is available again. If you're using OneNote at work, you can request more storage from your OneDrive for Business or SharePoint administrator.

You can then consider splitting up a large notebook section into multiple ones or purchasing more storage. If you do reach your file size limit, OneNote will notify you. You can still reference such content by inserting a link to it instead of directly embedding the content in your notes. Although it can be hard to tell exactly how much content has been added, especially if you've shared your notebook with other people, you and your collaborators can keep some obvious file sizes in mind - for example, by not inserting multiple video files that are several gigabytes in size into the same section, or generally not overloading sections with unnecessary media content. These limits determine how large your notebook sections can get, so you’ll want to make sure that you don’t reach or exceed this limit. On SharePoint and OneDrive for Business, the storage limit is configurable by the administrator, but it will typically have a limit of 15 GB or less. For example, on a personal OneDrive account, the free storage limit is 5 GB.

Both OneDrive and SharePoint have file size limits that can impact the maximum size of your OneNote sections. OneNote stores your notebook sections as individual files. However, even when OneNote handles all of the syncing of notes between your computer and your devices, there are a few best practices you can follow to make sure you have the best experience. This means you should never need to manually move files around, like you might be used to with traditional computer files in File Explorer on a Windows PC, or in a Finder window on a Mac. No matter what version of OneNote you use, your notebooks are designed to be saved and synced automatically by the OneNote apps themselves.

Fix onenote sync issues