In some cases participants will run into a situation where their debit or credit cards are being declined, even in cases where there are sufficient funds.
Chances are your bank is triggering this for a variety of security reasons:
You're making a large online purchase
You're making a purchase from a foreign country to a U.S. based company.
To get around this, contact your bank or credit card company and let them know you're trying to make such purchase and they will authorize the payment.