The eurostar website is a right pain to book with! Prices change constantly and trains appear then disappear! When we booked to go in december, we booked the direct trains, however we looked a couple of weeks after we had booked and the direct trains had gone from the listings! The direct train only came back on the website a couple of weeks before we left! It really is hit and miss. And the prices change alot depending on when you book, very usually the prices go down the nearer you book to the time you go, although we have seen the opposite happen too! Also, last time when we booked, the train was listed, but standard class was sold out, so we could only go first class. I do not understand how standard class could have been sold out 3 in advance and it was during the school term time! Alas, we looked a couple of weeks before we went, and standard class had come back on for sale. Oh and if you try to email eurostar, they are no help at all.
So i guess what i'm saying is that all you can do is wait if you want to go direct because it will be listed eventually! And keep checking everyday!