I have working on AEM 6.4. User synchronization is working fine between two publishers with Sling Content Distribution , but UGC or any content not get sync to other publisher.
Can you verify the configuration as described at 
Also, setup a DEBUG logger on the following classes to get more info on the failure:
 User Synchronization
Did you follow all of the details here?
Apache Sling :: Content Distribution (org.apache.sling.distribution)