Baked Caramel Popcorn

Several times, I have made great caramel popcorn using this recipe:

• 5 quarts popped popcorn
• 2 C. brown sugar
• 1 C. butter
• 1/2 C. corn syrup
• 1 tsp salt
• 1/2 tsp baking soda

But the last two times the caramel turned into a powdery sugar after baking. Does anyone know what I did or what I can do to rectify the problem?
Thank you.

Bob

By Bob from Wilmington, NC