Mate. Mate. Mate.
There is only 2 D+D's.
One is a popular fantasy role playing game. The other runs the place.
And uhhhh, how can the show creators stick to the source material when they have passed the source material and are foraging new ground?
"Foraging" new ground? Are they out picking mushrooms in the woods? I think you mean "breaking new ground".
It's pretty simple mate - the story, the character arcs, the plot development: all were better in the earlier seasons, when they were mostly using GRRM's source material. When they started cutting chunks out and re-writing stuff it lost some quality. They deliberately left stuff out and went too fast in order to get ahead of the books, and now they are struggling to write as well as GRRM does. In hindsight they should have gone to twelve, or even fourteen episodes a season after season three, used the extra screen time to go more in depth using the source material and NOT raced ahead.
Don't get me wrong, I enjoy the show greatly. Some individual episodes have been excellent, and there are individual scenes in almost every episode that have been done sumptuously. Some of the character arcs have been brilliant... but the books are just better in terms of consistency, complexity and depth.