The majority of the time when you see post with some number of messages with a zero backlog, it's an indication that a long running transaction is running on source without a commit or rollback. Thus, post is waiting for a commit or a rollback.
This is normal expected behavior.
Have the user on the source either issue a commit or rollback. You can query the v$session table on source to see if any connections match the age of qstatus on target. If the issue is not related to an uncommitted transaction, call support to determine what the messages are in the queue.