first reason is probably that the frosting isn't thin enough, consistency wise. Also, if you use Crisco, since they took out the trans fats, icing tends to not want to stick to the cake. (is there anything that has shortening in it that is even remotely healthy, so why take out the trans fats 
--sorry- personal pet peeve)
many here recommend the store brand shortenings - unlike name brand Crisco, these were not reformulated to remove the transfats. there are also recipes using hi-ratio shortening, but this isn't a supermarket product, at least where I live, so I would try the store brand all-vegetable shortening first.
Just thin your icing with a tiny bit of water. Also I recommend using the Icer (#789) tip in a 16" or 18" bag. The icing is squeezed out in a long strip about 1/4" thick by 2" wide. After you cover the cake with the strips, you gently smooth the lines together.
I add corn syrup to my frosting and since doing that I have not had the "not sticking" issue.
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