in this case, a file that could be lacking permissions to sync or the like. 

      Click the blue cloud at the bottom right, then click the sync issue it is regarding, example:

It will then explain what is happening. The example below is because the file doesn't have permission to sync, which is fine with me, I don't need that file syncing, though it will always appear with a red x for me.

Example:


If it is a file permission, you need to right click the file in question, click on .

Click  Tab.

Click:

Click your user ID Example:

Click the allow check box next to Full Control:

Click Apply > Ok > OK and this will allow the file to sync if it is this issue.