If you're trying to find advance fares, first of all check that you're not trying to book too far in advance (as in, before the advance tickets for the date you want have gone on sale)
How far ahead you're allowed to book today is called the "booking horizon", and can be found by clicking "booking horizons" at the following link:
http://www.atoctravelagents.org/
(the url of the actual booking horizons page changes every time they update it, which is annoying as it prevents me from just bookmarking a specific page that'll always be up to date)
If they
are currently available, then I usually find its easiest to trawl through all the possible trains on a given day at
http://www.nationalexpresseastcoast.com - we may be annoyed with them at the moment for defaulting on the east coast franchise, but they have the best booking system I know of. They can sell tickets for anyone's trains, not just their own and if you're worried that they'll go bust or cause trouble you can book the tickets elsewhere after using their search engine to do the legwork and find the cheapest trains.
If you're struggling to persuade any of the sites to sell you an advance fare for the whole journey, check what changes need to be made along the route and book advances for each individual train: - sometimes if a route uses a rubbish (ahem Northern rail) train that can't have reservations or advance tickets for even part of the journey it might not show up for the whole thing.
So for example, if you wanted to go from Birmingham to Blackpool, book the Birmingham to Preston ticket as an advance, and get the rubbish Preston to Blackpool ticket thats full price, or even just buy it on the day.
Hope this helps.