I cannot figure out why my fondant is cracking, any suggestions?

Is it homemade MMF? If so, likely too much powdered sugar and not enough liquid. I would add a bit more liquid....you can keep kneading it with crisco too - that often helps. If it's Satin ice, you can't leave it exposed to air for even a few mins. Mine cracked on me too. If it's Wilton, then I'm not sure....

Some helpful hints from Lindy Smith:

http://www.lindyscakes.co.uk/Blog/2009/02/27/why-does-my-sugarpaste-crack/comment-page-1/

HTH