In Super Mario Galaxy 2, you have to fight Bowser Jr. who is in a giant robot suit sort of thing. You have Yoshi, and there are Bullet Bills flying at you. I've figured out that I have to get Yoshi to eat the Bullet Bills and spit them out at the blue domes on the front of the robot suit (twice on each side to fully destroy each one), but when it comes to hitting the one on the robot suit's back, Bowser Jr. turns around too quickly for me to be able to shoot him if Yoshi eats one of the Bullet Bills that come out of the robot suit's back.
Is there something I am missing here? I keep dying, and would love to be able to make it to the next world.