Assassins Creed 2 Glitch!! Practice makes Perfect Sequence 2 PLEASE HELP!!?

Assassins Creed PC players ONLY! I dont know if this is a glitch or not, but I can't fight the guy after uncle mario talks to me. It says that I have to lock on to the target and press " mouse button 1" and I do but nothing happens. Please help because I really wanna complete this game! Thanks :)