About one in five people did not redeem the £66 energy support voucher they were sent by PayPoint in November, the company has said.
PayPoint sent out hundreds of thousands of vouchers in November under a Government support scheme.
But only 81% of those vouchers had been redeemed on Sunday when they ran out – 90 days after they were issued.
It means that thousands of households with prepayment meters have missed out on energy bill support they were entitled to.
The redemption rate is still decent, PayPoint said, and is broadly comparable with similar schemes, like the warm home discount.
But if similar rates are seen across all issuers of vouchers it will lead to households missing out on several million pounds between them.
The vouchers are part of Prime Minister Rishi Sunak’s promise in May to give £400 to every household to help them with their energy bills.
For most households this amount was simply deducted from their direct debits. However customers with prepayment meters had to redeem vouchers.
Steve O’Neill, corporate affairs and marketing director at PayPoint, said: “Last November’s EBSS (energy bill support scheme) vouchers have now expired and overall redemptions are consistent with October vouchers at 81%.
“Our 28,000 retailer partners across the UK have once again done a fantastic job in supporting their local communities and delivering this vital support.
“Final redemptions of over four in five recipients is generally considered a good result for Government voucher schemes, based on others like the warm home discount.”
He added: “A PayPoint survey of 2,000 adults using its services over the first three weeks of January showed 93% found it easy to redeem their EBSS vouchers at a top-up point.
“It also found 91% of people wait two weeks or less to redeem EBSS vouchers after receiving them, while 81% wait one week or less.
“Almost a quarter – 23% – said they wait less than a day.”
PayPoint said that, so far, 79% of customers have redeemed their December vouchers. The rest have until March 8 to do so.
Meanwhile, the 28% of customers who have not yet redeemed their January vouchers have until April 9.
Rocio Concha, Which? director of policy and advocacy, said: “As an urgent priority, the Government and energy suppliers must work together to share best practice and quickly improve the scheme to ensure that all customers on prepayment meters – who are more likely to be vulnerable and on lower incomes – are able to access this vital support without further delay.
“The Government should also set out a clear process for the reissuing of any expired or lost vouchers to ensure this works well for customers.
“If you are on a traditional prepayment meter and haven’t received your vouchers, or are unsure how to redeem them, you should get in touch with your supplier as soon as possible for more information and to make sure they have the correct contact details.”