Adding money to account from pre-paid debit card not working

I have been having a hell of a time getting my debit card to work with T-mobile, keeps saying my card is denied. It is a Walmart prepaid debit/credit card that they give to employees or you can buy from their Money Center in Walmart. What is the problem? Everywhere else I go they gladly accept my card, this is the only place that I'm having trouble.

Cannot purchase an activation kit for Nexus 4 phone, cannot pay for 4g broadband with T-mobile, almost seems like T-mobile does not want me as a customer. :/

Any help would be greatly appreciated.