Users of a popular prepaid card sold at Walmart stores have suddenly run into trouble checking balances and accessing their funds.

Frustrated consumers took to Facebook and Twitter Wednesday to air their grievances, with some complaining that they had been unable to access their funds for two days or longer.

The complaints involve the Walmart MoneyCard, a reloadable prepaid card issued by Green Dot Bank.

In an emailed statement Wednesday, Green Dot said that its new transaction processing partner, MasterCard Payment Transaction Services, was experiencing slowdowns in some systems. Green Dot acknowledged that customers could not access their card balances online or over the phone, but said that they could still check balances by mobile apps or text message.

It also disputed claims that users could not access their funds.

"There is no impact to the ability to make purchases or get cash from ATMs," Green Dot's statement read, "and reloads are all posting properly. We understand the important role we play in the daily financial lives of our loyal customers and apologize for any inconvenience this may cause."

But Barbara Martinez of Tornillo, Texas, said that her card had been declined several times over the past two days.

"I knew I had money in there, so I was wondering what was going on," she told American Banker on Wednesday.

Martinez added that she postponed a doctor's appointment that had been scheduled for Wednesday out of concern that her card would get rejected when she tried to make her co-payment.

Brian Pettis of Sedalia, Mo., said that his paycheck typically gets deposited on his prepaid card each Wednesday at 11 a.m. He was relying on that money to buy lunch, but the card was declined multiple times, he said.

"Going all day not eating wasn't really ideal," Pettis said in an interview.

Green Dot announced in March that it had migrated approximately 50 million account files to the MasterCard Payment Transaction Services platform, and that it would migrate 50 million more later this year.

It is unclear how many accounts had been affected by this week's glitch. Some social-media users reported that their cards were working on Wednesday afternoon.

Walmart, of Bentonville, Ark., did not respond to a request for comment.

The problems come at a poor time for Green Dot. The firm is locked in a pitched proxy battle with a large shareholder, Harvest Capital Strategies, which is seeking to oust Green Dot Chief Executive Steve Streit. The Pasadena, Calif., company's shareholders are expected to choose between two competing slates of board candidates at Green Dot's annual meeting Monday.

The complaints being lodged against Walmart and Green Dot are reminiscent of gripes that were voiced last October by users of the RushCard prepaid card.

That incident, which followed a change in payment processors, affected more than 442,400 consumers; users were unable access their funds or pay their bills, in some cases for several days. RushCard's parent company recently agreed to pay a total of $20.5 million to settle a class-action lawsuit brought on behalf of customers.