Damien

Part of the problem is that the pacing was too slow when they were still adapting the books which meant as soon as they left the books they suddenly had to bring it all together within a few short seasons. The books had left them with an ever-increasing character and storyline count, people located all over the map and too many mysteries which they had to resolve themselves.

It's clear George RR Martin isn't have an easy time bringing altogether either otherwise they would have had the final two books to adapt already.

Television is a different medium to television and they had too few concessions to this in the earlier seasons which they're paying for now. They needed to be more ruthless is limiting the scope of the books early on then they might have had an easier time closing down all the open elements of the plot.