Harry Potter And The Deathly Hallows: Part II Q&A

Professor McGonagall is using protego when it says that i shall press 5 many times, but i use the shift-button to cast protego.
so, what am i going to do? I can't get past the giant!
first: press 2, that works
second: press 5, but it doesn't work, and it look...

When i'm on the bridge, playing with Professor McGonagall, i can't get past the troll in the end. It comes up that i should press 2, then i do that, and afterwards it come that i should press 5, but it doesn't work.
How do i do it?