How to print screen on a Compaq ?

Okayy so i have a hp COMPAQ and well i know how to print screen but it dosnt work. I do the same on the other pc i have and it works. On the other pc I press prt sc+ctrl+alt and it works ive tried every thing on my HP and it dosnt work. Its pissing me off. Its right on top of num lock and it dosnt work with the numlock on or off. Any one know wht i can do to fix this problem :) ?