I was reading about Ina Garten's Caramel at Foodnetwork. One poster replied to others that if you're having problems, it could be because you're using beet sugar and not cane sugar. To quote her/him:

"For those of you having problems with the caramel crystallizing and not working at the end, you might want to check and make sure that you are using cane sugar, NOT beet sugar. They're supposed to be exactly the same, but there's something like a .5% difference in their chemical structure which makes it impossible to use beet sugar to make candy. I had frustrating failures with making toffee until I did a search on the internet and found out that beet sugar was the culprit."

I know C&H is cane sugar; not sure which other brands are.