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.