When, using entitydata, you tried to change the rotation of a mob or the Pose of an Armor Stand, you can see the changes only if you have relogged in the game.
I've used this command: /entitydata @e[type=ArmorStand,r=4] {Pose:{LeftArm:[45.0f,0.0f,0.0f]}} or /entitydata @e[type=Zombie,r=4]