qualifying categories not working as expected

Hello, I am using v6.5 and when i try to setup promotion on qualifying categories it is not working as expected. For all products in the category, when i try to add the product to cart, it should ideally apply the action(% off). But its applicable for only one product and not on the other product of the same category. For eg: consider prod A and prod B which belongs to cat A. The promotion which i want to achieve is buy 12qty of products from cat A and get 10% off on the products. When i try to add '6' of prod A and '6' of prod B then this promotion is not getting applied. But when i try to add '12' of prod A then only this promotion is getting applied. So how can i achieve the expected behaviour ? Any inputs are welcome.

Cheers, Shen.