I had a similar experience with using the $10 off coupon. I did seperate transactions in the self check line, total was around $13 and the coupon wouldn't take. The person working the self check would come over and manually put in the $10 to take the bill to $3 and then put the coupon in the garbage. Did this this three times on the same visit, something to try out if you have issues as well, ymmv.