When you are told they can't accept them, ask to see a copy of their coupon policy. It states that they accept them as long as they are scannable, have exp. date, a mailing address etc. They should take it as long as it scans.
I personally haven't encountered any problems at my walmart, but I have read of several (on afullcup) that have and they just ask about the coupon policy and they always change their tune. If necessary ask to talk to a manager and ask them for the coupon policy. It is posted in customer service at most if not all walmarts.
OR...
Go to walmart.com and it is posted on there and you can just take a copy with you.
hth!