I have selected a screen saver but it is not appearing. Why?

Actually, I have bought a MacBook Pro just some time back and I am just learning to use it. I have a problem with my screen saver. Yesterday, I was talking on skype with some one and during that time my screen saver was appearing just the way I have set it. But suddenly today the screen saver has stopped appearing and I realized it when I was reading something on my Mac. What happened was just that the display went to sleep and after some time my computer went to sleep (just the way I have set it on) but the screen saver did not appear at all. I have selected the screen saver many times but it is not appaering. 😟

Can someone please tell what could be the reason for this? Coz I have tried it now many times but it is not working, though I have not changed anything at all in the system.