I think its very normal for a series to somewhat slow in the middle - I see it with a lot of book series where book one blows you away - book two trudges along and then book three blows you away again for the ending. With trilogies its not so apparent, but with a longer series like ASOIAF (esp when the time between books is long) the slower middle ground appears to be far more extended as it just has more ground to cover - although longer series can also throw in a middle peek book (eg look at Malazan Book of the Fallen and Memories of Ice)
Of course the risk is that the author gets stuck in the middleground rut and fails to fully pull themselves out of it. I think GRRM really recognises this problem however and I think he will pull himself out of the doldrums - we just have to let him get through it.
This is a point worth exploring. Personally, I never complain about a middle book (I liked AFFC a lot), because I am always good at seeing the big picture in a long series like this one (my favourite three series of all time (in order):
Wars of Light and Shadow (9 volumes as of this fall, projected 11, all big and fat);
ASoIaF, currently 5 fat novels, with at least 2 to come;
Malazan Book of the Fallen (10 volumes, plus the Esslemont books taking it to 13). In a long series, you cannot relentlessly build the tension without a release, and building the tension too early is fatal. If GRRM tried to wind this story up too soon, you'd get your climax, but it would not be satisfying. I suspect that there will be a lot of happy readers when
A Dream of Spring hits the shelves (in about 10 years). The middle part of a long epic is there to hone the various plot lines and make an ending possible. Picture a story starting as a single line, then branching multiple times, and then converging again to two or three large lines just prior to resolution. We are now at the point where convergence has started in earnest, which will be continued in
The Winds of Winter.
The middle books in a series are what makes the ending so darn good. Complain if you will about the story not being resolved (a truly justified complaint in
WoT), it is this context and complexity in the middle part of a long epic series that make the denouement so darn powerful. Remember, LOTR, which most of us love as a masterpiece of fantasy, was really only one book (the publisher insisted on splitting it, not JRRT), and it was as long as one of the GRRM books. Politics, except for a bit of RotK and at the Council of Elrond, did not figure at all in the plot, whereas in ASoIaF, it is the center of the story. It is a much bigger and more ambitious work than LOTR (though GRRM doesn't hold a candle to JRRT in world building. No one has really approached JRRT's heights in world-building).
Book five and loving it, and ready for the next. Time for another re-read yet?