I agree, Walmart can be a pain in the arse about coupons.
Once I had a cashier mad at me for not writing the prices down on paper before she scanned the coupons, once a cashier refused to let me pay the extra $1 on a product that was free because, she said it wasn't allowed (I was trying to use my Bryans bologna coupon on a large roll of bologna to save extra money), some refused the coupons that if you buy 1 product you will get a 2nd one free...
I have never been able to use a free coupon on a B1G1.