"It was embarrassing, everyone was looking at us," Susan Bleau says of the moment she tried to spend free school meal vouchers worth £45.
After weeks of delays, her daughter's school had finally received the vouchers and printed them out for her.
But at the till, with a trolley full of shopping, the vouchers failed to scan.
Edenred, which runs the government scheme to give pupils in England vouchers worth £15 a week, says all of its vouchers are valid.
But ever since it was set up, in late March, the scheme has been beset by problems.
The system was rebuilt over the Easter break but the improvements were slow to take effect, with schools still struggling to log on, parents unable to download the vouchers and some even saying the vouchers failed when they tried to spend them.
With the newly printed vouchers in her pocket, Susan and her 11-year-old daughter had taken the bus from their home in Wembley, north London, to Tesco at Brent Park and queued outside before picking out what they needed.
"We had everything - cheese, pizzas, yogurt, smoothies," Susan says.
But when they tried to use the vouchers to pay, they would not scan.
As the queue built up behind them, the cashier called the manager, who tried to enter the barcodes manually before pronouncing them faulty.
All the shopping had to go back and the pair had to leave empty-handed, humiliated and disappointed.
"You're not the first and you won't be the last," Susan remembers one of the supermarket staff saying.
It was a Saturday, so all Susan could do was leave a message on the school voicemail and wait for replacement vouchers the following week.
"We had to live on what we had which, wasn't enough but we had to cope," she says.
Raphael Moss, their head teacher, at Elsley Primary, who knows of at least one other family this has happened to, says: "I can't imagine how distressing and embarrassing that must have been."
Tesco says it is not aware of widespread issues when customers redeem their vouchers in its stores.
And Edenred insists every supermarket eGift voucher it sends to parents is valid.
The company suggests poor or damaged printed copies could be responsible for some of the failures. But many school leaders believe the problem is not uncommon.
"This has happened to so many of our parents," said one head on Twitter.
And Martin Knowles, head teacher of Essa academy, in Bolton, says about one in five of parents in receipt of the vouchers has had problems at the till.
He says the school is considering abandoning the Edenred vouchers and setting up its own scheme, at a cost of £8,500 a week, if the problems continue.
Chiswick School, in west London, continues to hear from parents whose redeemed vouchers do not work at the tills, according to school business manager Danny Sohal.
Other school leaders, from London, Cornwall, the West Midlands, and the north-east of England, have also experienced problems.
Edenred says it continues to make big improvements to its system and waiting times for schools and families to log on to the site have been "almost eliminated".
More than £52m worth of eGift cards have been issued to schools and families since the scheme was launched, with 16,500 schools signed up, it adds.
The Department for Education says it is encouraging schools to make their own arrangements and to use Edenred only when that is not possible.
But many heads are worried about doing their own deals to provide food or vouchers, whether with local catering suppliers, direct with supermarkets or with other voucher specialists such as Wonde.
They believe their only option is to use Edenred, as it is fully funded and, if their budgets are in surplus, they will not be able to claim back money they spend on alternatives.