Changes to issue data (comments, attachments, fields) are not being synchronized, and no errors appear in the troubleshooting screen.
You're probably making the unsynchronized changes when logged in as the synchronization user for your synchronization.
The synchronization user is a special user in your JIRA system that is used exclusively by Backbone to read and write issue data. Backbone uses this user also to identify which changes should be synchronized – so manual changes with the synchronization user will never be synchronized.
Log in as a different user, and your changes should be synchronized.
If this does not help you, please contact our support team.