An issue does not get synchronized and appears in the troubleshooting section with the category 'Missing message' and a message like 'A previous message for this issue was not yet processed'.
Whenever Backbone queries for changes, it generates a message file for each issue with changes. Every message contains a sequence number which guarantees that the messages are applied in the right order at the receiving side – this is especially important for distributed configurations. If you see a missing message error in the troubleshooting, a previous message has not been processed yet.
Look for other problems for this issue in the troubleshooting screen, and fix them.
If the problem still exists, perform a resync in the sender project.