I've heard and experienced that many cashiers aren't familiar with their own store's coupon policies.
This can be frustrating when you have your stack of coupons ready to go and it all comes to a screeching halt. You've got other folks behind you in line giving you the evil eye and you just want to give up and go home. Well, don't! Keep a copy of the store's coupon policy (generally found on their website) in the back of your coupon caddy, envelope, etc. The cashier will either become more knowledgable about policy, or will be so intimidated by the fact that you know their coupon policy, chapter and verse, that he will just put your coupons through out of sheer embarrassment! If that, too, fails, then ask to have the store manager ring you up at customer service.
Here are links to some stores' policies.
Kroger (Digital Coupon Policy)