Hi,
This turned out to be an issue with our DR site.
We were using a 3rd party company to host our DR site.
It all came down to routing and firewall settings.
Our main issues were getting our replications appliances to talk. Once that was working the sync was sitting on 0% and not syncing
The issue from here was the repplication appliance at the DR site was unable to talk to the host it was sitting on.
Once we had resolved this, all began to work.
We haven't had an further issues.
Not sure what would cause your problem considering it was just working, but it was firewalls and routing that caused our problems.