As far as using pre-made frosting I would not really recommend it due to the flavor loss. I find pre-made icings to have a slight chemical taste and I would rather have aause frosting with a slight crust than a chemical flavor.
I am just wondering... are you by any chance using the royal frosting recipe? This definately would become really hard by the time it's served and may definately crack. I also read that if you decorate a cake while still frozen you may have this problem because the cake will "breathe" while defrosting. If you think about basic science, heat expands and cold contracts; If your cake is frozen when you frost it you stand a chance that when the frozen cake expands it will crack.
Advice? check to make sure your not using royal icing and ask someone who is in your area, like a wilton instructor, to see if the humidity or lack thereof in the area has an affect on the buttercream recipe and what can you do to compensate for this problem.
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