Martin has allowed the story to twist out of his hands. It's bigger than his original plans for it, and he can't seem to cope with the incredibly enlarged (and in my eyes unnecessary) scope. The story of Westeros and it's succession for a King is interesting enough in it's own right. Adding in Daenarys' storyline in was a huge increase in scope that was difficult enough on it's own, but now he's also throwing in Arya's storyline exploring a new culture, that guy from the Iron Isles who is trying to be big and bad while balancing the original succession story. This doesn't even mention what is happening beyond the Wall and whatnot.

I get the impression that the original story was just the Westeros succession tale, but with all the extras I just mentioned that probably were added on the fly, his cast is so huge and the amount of things happening is so great that he has just lost control of it altogether, or simply lost interest in it. FfC was fucking weak in comparison to the first three books, which were really, really well written (my favourite being Storm of Swords). I think he just let the project spiral away and doesn't care about it anymore, which is a disappointment.