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).
The reason it worked on the smiling Titan is because she was once dina fritz who was royal blood line which passed it down to Zeke and Eren kills her do to that fact it doesn't work on rod is because he was born of royal blood but didn't hear Eren for he rub his ears off. Therefore Eren has a founding Titan in him which makes him be able to us the means to control titian's for the royal blood line also goes through him since is dad ate Frida reiss and he ate his dad but that's all I got