You might not be compressing the mixture enough. RTK is sensitive to humidity as well. I had the same trouble with a teddy bear and I used toothpicks going in different directions and added some royal icing for glue. After the fondant was added it held up for the event. However I warned the servers about the toothpicks because they can be hard to spot and dangerous. I hope this helps and your piece turns out OK.
10/25/10 at 9:06am