My normal go-to site for most European train booking is
http://loco2.com. They
tweeted a few days ago saying
IT issues at Deutsche Bahn mean some London tickets are not available for sale. Check your search and contact us
[email protected] if you need
So, looks like it's a blip
In general though, for the best prices on these, you need to book 88-92 days out. You can't generally book >92 days out, and within a few days the very cheapest tickets will have gone...