The da vinci code Level: Temple church

Hi

I managed to get until the second check point on this level. After downloading several walkthroughs and doing EXACTLY as all suggest, meaning:

"Now, go to the door and examine it. There's a gap below the door, and it seems like you can slot something thin through it. Piece of Card-board, man! Use the cardboard at the gap, and you'll slot it through. "

the game refuses to do so ... it might be a bug ... pls. help.