Thousands of savers have been waiting up to three months for Nationwide to move their money to a top-paying account.
Britain's biggest building society was deluged with applications after launching a market-leading cash Isa on March 17.
Some 250,000 savers rushed to open the fixed-rate deal within the four weeks it was on sale.
Backlog: Nationwide was deluged with applications after launching a market-leading cash Isa offering a top 0.75 per cent plus cashback on March 17
But a backlog means more than 2,300 have been left in limbo, waiting for their accounts to be opened.
The Isa, only open to existing customers, offered a top 0.75 per cent rate for 18 months.
Savers could also pocket an extra £50 cashback if they transferred at least £10,000 from another organisation — effectively raising the rate to nearer 1.1 per cent.
But demand from savers battling record-low rates took Nationwide by surprise.
One such person is Money Mail reader Carol Waldron, 71, who visited her local branch in Stratford-upon-Avon in Warwickshire with husband Ray soon after the Isa launch, so they could both open an account.
They were told they could not submit applications in branch as lockdown rules meant only essential services were possible. Instead, they were sent away with forms to open the accounts by post.
They filled these in on March 24 and posted them to Nationwide along with two cheques for £10,000.
Yet when Carol called the building society to ask about their applications two weeks later, Nationwide said it had no record of receiving them.
She was told to cancel the cheques and reapply. But a week later, on April 14, it tried to use the original forms and cheques to open the accounts.
This would not go through as the cheques had already been cancelled. And despite receiving the new paperwork, the couple were still waiting two months later.
It was only after Money Mail intervened that Nationwide resolved the problem and backdated the interest to when they originally made the application on March 24.
It has also offered £100 compensation to both Carol and Ray, along with an apology. Carol, who has been a Nationwide member for nearly 20 years, says: 'We spent time and effort cancelling cheques, visiting the branch and endless phone calls trying to open this account. I expect better service.'
Other members have taken to Twitter to complain, asking: 'Why has it taken two months to process my Isa?'
A Nationwide spokesman says: 'Our Member Exclusive Fixed Rate Isa offered a market-leading rate which led to very high volumes of applications.'
Under Isa rules, banks and building societies should transfer a cash Isa within 15 days. But an industry-wide voluntary agreement aims to complete 85 per cent of transfers within seven working days.
If your transfer is delayed, the interest on your new account must be backdated to the date on the cheque or from day 16 after the transfer request, whichever is earlier.
Nationwide says the £50 cashback will be paid by June 30. If the switch is not completed by then, the payment will be made in July.