Does anyone else think that Martin isn’t really sure what is going on himself? The story lurches from ill-conceived sub-story to a good sub-story and back again. He creates and kills of characters at random and generally seems to have no story arc planned. That said, given he didn’t really expect to get past the 3rd book, maybe that’s normal. It does read a bit like that TV show ‘Lost’ at times, narrative on the hoof. I think he shanked it into the weeds in book 4 and hasn’t been able to hack it out successfully since. It must be next to impossible to sit down, pick up the pen and plan out a story arc for 6 or 7 massive novels – Marcel Proust did it but that’s a singular case, anyone else is going to have the narrative go pear-shaped at some point.
Writing these big fantasy epics was never GRRM’s style, if you read what he was doing beforehand, so I think he just badly ran out of steam with it after book 3 and that did the damage. I think they’re still good reading on a page by page level – he’s great at character development, dialog etc. It just gets a bit of a chore if you don’t really believe that the author is in control of his story.