As regulators prepare to strengthen rules for prepaid cards, lawmakers are pressing Walmart, MasterCard and Green Dot to explain what went wrong last month when shoppers using Walmart prepaid debit cards were unable to get access to their accounts.

For several days in May, some users of the Walmart MoneyCard were not able to view their account balances or activate new cards. Customers also reported that their cards had been declined for purchases. The cards use MasterCard’s network and are issued by Green Dot, one of the largest prepaid card providers in the United States.

The failure echoed a technical breakdown last year that cut off thousands of customers from funds they had loaded on RushCard, another popular prepaid card. Last month, RushCard agreed to pay $19 million to settle a lawsuit with cardholders over the problems.

Prepaid cards can be a financial lifeline for their users, many of whom are lower-income workers or, for a variety of reasons, do not have bank accounts. Some cardholders have their paychecks automatically deposited to their cards and use the accounts to pay rent, utilities and other essential expenses. When the cards fail, financially vulnerable customers can be left stranded.