Oh sorry, I did not know the kiln was only 13 inchs deep. You could try a baffle tooo so that the sides are not heating up tooo fast. Are you using different colours. What is the thickness. Hope we can help you out. (and why did the first ones work, maybe check under polorized film just to make sure. What did the break look like. Did it break on the way up or down. Did you Peek!!!!
(I am sure you did not!!! but more info is always super for the investigators!!!! (or helpers!!! tee Hee!!!)