I found the problem for anyone who is interested. It’s the German DNS blocking issue. In January Germany started blocking sites like Demonoid from resolving on German ISP DNS servers. Anyone in Germany can get round it by using Googles 8.8.8.8 DNS or just use the IP address and not the URL. It looks like chinahdtv.org was added to the list a few days ago. As the seedbox I was using was in Germany the torrents stopped working. So just editing the tracker details and replacing the URL with the IP and all the torrents are now working.