You will receive a notification that your restore failed even if just one file fails to restore. This does not mean that your entire restore failed, but only the items that are listed in the message. Throttling is the most common reason that files fail to restore. Read more about throttling here.
If a file fails to restore while trying to restore a connector in place or while trying to restore individual files, we suggest you try to restore this specific file using selective restore (see: How do I restore a file or a folder?)
If a file fails to restore while trying to restore a connector to folder or while importing, try to restore this file to folder (this is only possible for Calendar, OneDrive, and Mail) or try to import it again.
If many files fail to restore, regardless of what method you used to restore or import, try the above step.
There are two cases when a restore fails but you don’t receive a list of failed files:
1) A restore can fail because of a bad request, for example, if you try to restore certain data types together. Make sure you restore Sites and Groups separately from Mail, Calendar, Tasks, and Contacts.
2) A restore can fail because of a bad token. Make sure that your credentials are valid by checking the state of the connector.