If you are using Windows 7 or 8 (in some cases) you are probably using the "older" sync client for OneDrive for Business. There are a few things that impact syncing between the OneDrive client and the cloud. It can be something as simple as your file is still open somewhere (Did you start it at home and leave it open and now have a second one open at school?) or your file names are too long and contain prohibited characters. (See this article to check.)
Microsoft has a longer-form article on this page but for most people you need to check these options first.
If you are copying complex folders (folders in folders in folders) the first issue may be the problem:
Make sure that your file sizes, items counts, and file path lengths are within the limits
If a file you’re trying to sync is too large, or if you’re trying to sync too many items, OneDrive for Business sync might not work. Review the limits for the size and number of files that can be synced, and make sure the files you’re trying to sync don’t fall within the listed restrictions or limitations.
If you are still having problems try to:
Repair a OneDrive for Business sync connection
(From Microsoft's: repair a sync connection article:)
Get started by right-clicking the OneDrive for Business menu icon in the taskbar notification area, and then clicking View sync problems.
An error dialog box opens that describes any current sync problems and suggests actions you can take to fix them. Sometimes, you might prefer to just clear all sync errors at once, or you might see this error:
“An error occurred while attempting to synchronize this document library”
In either case, you can run Repair instead of trying to fix each sync error one at a time.
Finally, you can: