Cart total incorrect after applying coupon

I think I've set up something incorrectly but don't see it elsewhere in the forum.

I've created a 10% discount for category A. The discount is applied correctly to an item from category A when placed in the shopping cart. Ex. Subtotal $100 - discount $10 = total $90. Zen Cart also correctly does not apply the 10% discount if an item from category B is placed in the shopping cart. Ex. Subtotal $200 = total $200

However, if I have an item from category A and B in the shopping cart, the 10% discount is applied to category A and the cost of the item from category B is not added to the total purchase. It is listed in the shopping cart and the subtotal. Ex. Subtotal $300 - discount $10 = total $90

Any help is greatly appreciated!

I'm using Zen cart 1.3.5