There would be plenty of reasons if OneDrive is not syncing. If you are running an old version of the app, then chances are that you might experience OneDrive sync issues. Additionally, you could have just run out of space on the cloud as well.How do I sync OneDrive with my Desktop?
File Explorer icons show you the sync status of your offline folders and files. It's in sync with online version. It's getting in sync. The version on your PC is out of sync. To find out why, go to the right side of the taskbar, select the Show hidden icons arrow, press and hold (or right-click) OneDrive , and then select View sync problems.