Assassins 2 Mem Blk 3 Help

Does anyone know how to get past the Chase the thief mission in Memory Block 3? Ezio realizes she has taken his blade and u begin chasing after her. I remain with her step for step all the way to the end. She runs thru the door and u can no longer move and it says "Mission Failed". Is there a trick or secret here? Not sure what im missing. Ive arrived at the end before her, at the same time, and after her and everytime it says mission failed. Can someone be kind enough to help me out?

You have to stay on the roofs, at the end you will drop down from a hole in the roof into the building, it's annoying but remember to always stay on the roofs, there's more missions with her.

Thanks so much.

No problem, I think hardly any of us got it on our first tries.

You have to get on top of the building that the guy is hiding in. Then drop down into it and you'll pass it.