It sounds as if you are doing all the right things. The one thing you don't make clear is if you have tested whether messages sent direct to the mail list via email reach the list.
It is possible you have not properly completed the subscription process for the list so that when you send via Nabble the messages are rejected - as they would be if you attempted to send via email as well.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.