They abandoned a lot of material already from AFFC and ADWD which they could've implemented into seasons 5 and 6 but they chose not to. No Victarion, no Young Griff, no Arianne, no Quentyn, no Euron vs the Reach. They butchered Stannis' storyline, Jaime's journey through the riverlands was like 2 episodes in season 6. No Lady Stoneheart. They just cut 2 seasons of material to go through with their own stuff. They didnt run out of shit, they just chose to do their own thing which then failed massively.