since the pandemic, network rail has been taking advantage of the quieter network to undertake larger engineering projects, often at short notice. the result is that several train companies have been unable to adhere to the 12 week booking horizon and in some cases (Avanti on the West Coast are a classic example) cannot meet 6 weeks. travellers from Liverpool are still unable to buy advance tickets for the day of the FA Cup final - the cynical will say it's Avanti ripping off the traveller, but it's simply that NwR haven't confirmed to Avanti which trains they'll be able to run.
best thing to do is follow Greater Anglia on Twitter,
https://twitter.com/greateranglia / @greateranglia and set yourself up with a ticket release date alert.