Large numbers of energy bill vouchers have not yet been cashed in, according to reports.
It means some households have not yet received the first discount on their bill - worth £66 - after energy prices rose on October 1.
Households are getting an energy rebate worth £400 in total over winter, spread out over six months.
But the Post Office claims only 60% of the vouchers it should have received have been redeemed so far, according to the BBC.
It is unclear if this is because there has been a delay in credit notes being sent out, or if some customers have decided not to use them yet.
The vouchers are only valid for 90 days - so households who have received them are being advised to redeem them before they run out.
Are you still waiting for the first £66 energy bill discount? Let us know: mirror.money.saving@mirror.co.uk
"We encourage anyone who receives an email or letter telling them to claim their voucher at a Post Office to do so as soon as they can," wrote Post Office chief executive Nick Read.
PayPoint, which also processes the vouchers, said it had only put through around half the discounts it had expected to.
Its boss, Steve O'Neill, told BBC 5 Live's Wake Up To Money: "Obviously, there may be a number of reasons why they haven't redeemed vouchers including the mild weather.
"What we just want to make sure is that for people who have received the vouchers that they are aware of where they can redeem it, and obviously when they need to redeem it by, to get the support they need."
Everything you need to know about £400 energy bill discount
The money is being split into six instalments - you won't get it in one lump sum.
You will get a discount worth £66 in October and November, rising to £67 each month from December through to March 2023.
This means by now, you should have received one payment worth £66.
If you're a prepay customer, you won't get the energy discount automatically.
You will be given discount vouchers or Special Action Messages (SAMs) normally in the first week of each month, issued via text, email or post.
Customers will need to redeem these at their usual top-up point.
Direct debit customers will receive the energy bill discount automatically - meaning you don't need to do anything to get the money off.
The cash will be applied either as a deduction to your monthly direct debit, or as a refund to your bank account after your direct debit has been taken.
If you don't pay by direct debit, the discount will be applied as a credit to your energy account each month.
The credit should appear as if you'd made a payment to your energy bill.
Smart prepayment meter customers will also receive the discount automatically each month.
The £400 discount doesn't need to be paid back.
Your energy supplier won't need to ask for your bank details to apply the discount - so be aware of scammers trying to contact you.
