I have found a problem arising from the use of apps such as "GO SMS Pro", which have the option to disable the SMS notification for other apps. This prevents SMSSync from being alerted to new messages, so they are missed, and not sent to the server.
To rectify this:
In GO SMS Pro, go to...
Ensure "Disable Other Message Notification" checkbox is UNCHECKED.
Figured I might save someone the head-scratching that I went through