Why doesn't Eren's Founding Titan ability to control other titans work on Rod Reiss' titan? In Season 3 Episode 9 of Attack on Titan, Eren tries to use his Founding Titan powers to control other titans on Rod Reiss' titan but he can't. Why doesn't this work?
I don't think there's a spoiler-free answer to this.

Warning: Spoilers ahead


   You need royal blood to use the Founding Titan's ability, unfortunately Eren is not a royal descendant. When Eren first used this ability it was when he came in contact with the Smiling Titan (how she had royal blood is another spoiler).