Why is the mission control not working on my MacBook Air?

When I first got my MacBook I realized that you can take your three fingers (or maybe two?) and swipe towards the right and your Dashboard would slide over. Then I realized that you can press F3 (the mission control button), then add a new desktop, called desktop 2. It is kind of like adding a new page on the iPod touch while on safari. Once you had a new desktop on the MacBook, you could slide your fingers again and the new desktop would slide over.

I loved this function because I would have one desktop for personal stuff like facebook, tumblr, fluther etc. then the other desktop I would use for homework things. I really kept me on track with my priorities. Then all of a sudden, sliding my fingers wouldn’t make the dashboard pop up…. What happened? Can I somehow change this in preferences?