I deleted the repository in Github and recreated it again from my local copy. I also deleted the repo on Gerrithub and re-imported the newly created repository.
It all works again now. New commits are being replicated to Github through Gerrithub.
Not sure what the problem was but I have experienced it many times and I wonder if there is something I am doing wrong that breaks the replication from Gerrithub to Github.
Grateful for any suggestions.
Thanks, John.