Well, he did say that all the press/public events he had to attend because of the fame of the series were inhibiting his time, mainly, and ability to write.
Maybe it's not a writer's block, maybe the things that are happening are on such a large scale it takes up a lot of his time. As the paths of all the protagonists divided, he has basically several books to write, then cramp them all into one volume. Think of it - Night's Watch, Bran beyond the Wall, Stannis and battle of Winterfell, Littlefinger and Sansa in the Valley, Rickon on Skagos (I think), King's Landing, Aegon and Griff and their campaign through the land, Daenerys in Mereen, Arya in Braavos AND Arianne and the Martells in Sunspear.
I have a headache just thinking about it.