The Flying Key

I need help!!!! How on earth do I catch the Flying Key???? I beat the Devil's Snare part and go automatically to the room with the flying keys - I fly around going through the star/ring the flying key leaves and the bar on the screen slowly fills up - sometimes Harry goes very slowly and others he goes faster - I cannot get any further. When the bar is full it slowly starts to empty and I just keep flying around and around - never getting to actually touch the key. I have the walkthrough from gamefaqs - it's a lifesaver -but it just says this is like quidditch. Well, we usually lose at quidditch -maybe we're not doing it right?
So, A makes him go, the directional arrows control his direction - B seems to do nothing - HELP PLEASE!!!!!

you need to manuver your way around the moving keys to the one at the top!