Don't worry they release them in blocks, we had to wait ages for our return date to be released, they will phone you back when they are available.
The other thing to remember is that (unless you are booking late when it's really expensive) when it says "less than 10 seats available" it means seats at that price. I was tracking the prices before our dates became available and nearly had a melt down the first time I saw this message when the seats had only been out a day or two as we needed 5!
Also have they reserved your outbound seats for you? Although this only lasts a week or so they will keeep renewing it for you. There's a good website which explains about the different carriages and tricks for booking - I don't know if I'm allowed to name it and I certainly don't know how to do a link!