Why wont the minecraft website accept my card?

Hi, I just went to purchase mine craft beta from the website. the only thing is, i got denied. since i don't have a credit card, i went to walmart and bought one of the nonreloadable $50 visa card. i know they work online because i have made many purchases online with them in substitute for a credit card. ive tried three times with the walmart visa on minecraft today but it kept being denied. do any of you know why this may be happening?