Shoppers who use coupons in an extreme fashion are going to come up against brick walls on an annoyingly regular basis. Maybe it's a cashier who insists you can't use a manufacturer's coupon with one you got from the store, or a manager who is adamant that the coupons you printed online are unacceptable.
The trick to dealing with this sort of situation is to have the store's coupon policy on-hand and ready to go. Some stores' coupon policies are listed online, while others you have to ask for. Once you obtain hard copies, it's a good idea to keep them in your binder. Then if there is disagreement over the details of a transaction, you can show whoever is checking you out the relevant portion of that particular policy.
And again, always remain respectful and polite. You might find cashiers and even managers who are simply unfamiliar with their store's coupon policy, and by being friendly and helpful about it, you're much more likely to achieve positive results.