bar codes on coupons

I've been out of the whole "coupon scene" for about a year and a half now and I'm wondering about the printable coupons that only have two bar codes instead of three. A friend of mine said she's had trouble at walmart with the ones that have two barcodes. She said they aren't going through and so some cashiers are saying they can't accept them. Will all printables be changing to two barcodes and is there a known reason for them not going through? Thanks for any help and I hope it my question even makes sense. LOL