LEGO Harry Potter: Years 1-4 Nintendo DS Help?

I believe this is in Year 2, Chamber of Secrets where you have to collect all the stupid hats, where I get stuck. It's when you're down under the bathroom roaming around the corridors. There's a part in Free Play when you get to an area where you go back in time, and an area fills with water making a platform float. You jump on the platform and then jump to get the hat. My question, HOW IN THE WORLD DO YOU GET BACK ONTO THAT PLATFORM AGAIN?!?!?! I have to keep restarting the level, but nothing ever changes. Help please. I checked youtube, and couldn't find any help for Nintendo DS.