Millions of people have already received their $600 federal stimulus payment. But some Americans will have to wait a bit longer — and may have to file their tax return to collect it.

By law, the IRS said it must issue the stimulus payments by Jan. 15. After that, those who are entitled to a payment must instead claim it as a credit on their 2020 returns.

And some payments that have been sent are bouncing back to the U.S. Treasury. That’s because those payments were directed to inactive or temporary accounts that taxpayers can’t access.

It’s still not clear how many people are missing their money, but tax preparation company Jackson Hewitt said the IRS had sent payments to more than 13 million bank accounts that were no longer open or valid.

The bulk of the payments have already been sent out. More than 147 million payments, totaling $142 billion, have been scheduled: 113 million were issued through direct deposit (which presumably include the bounced payments), while 34 million will be delivered through paper checks and debit cards, according to IRS figures provided to the House Ways and Means Committee.

The IRS said the timing of the stimulus legislation, which was signed by President Donald Trump on Dec. 27, had created problems. “Because of the speed at which the law required the IRS to issue the second round of Economic Impact Payments, some payments may have been sent to an account that may be closed or no longer active,” the agency said in a statement.

Here’s how that happened: Companies like TurboTax, H&R Block and Jackson Hewitt sometimes set up temporary accounts for clients when they prepare returns. For example, clients who choose to have preparation fees deducted from their refund may be issued one of these accounts, allowing the tax firm to take its share and then pass on the rest. After that, the accounts are generally closed or inactive — but may still be linked to the taxpayers in IRS records. Payments that cannot be processed by the receiving financial institution must be returned to the Treasury.

A similar problem also delayed some of the stimulus payments sent out in April.

“It is disappointing that the IRS did not fix this problem, which it has known about for months, in light of the likelihood of a new round of stimulus payments,” said Lauren Saunders, associate director at the National Consumer Law Center. “People who need money now may have to wait months until they can file a tax return and get their refund, unless the tax prep companies are able to forward the funds.”

The IRS will not automatically resend payments until they reach the recipient, as it did in the spring. Instead, those who do not receive their payments must recover the money on their 2020 tax return by applying for the Recovery Rebate Credit, which can be found on line 30 of the 2020 Form 1040 or 1040-SR.

Individual adults with adjusted gross income on their 2019 tax returns of up to $75,000 a year are eligible for a $600 payment, and a couple (or someone whose spouse died in 2020) earning up to $150,000 a year will get twice that amount. There is also a $600 payment for each child for families who meet those income requirements.

Taxpayers can check the status of their stimulus payments with the IRS’ Get My Payment tool. If the payment was sent to an unfamiliar account number, it could be because the IRS sent it to one of those temporary accounts.

And if the tool shows “Payment Status #2 — Not Available,” that’s another indicator that your payment is not coming, the IRS said.

That’s the message Brandy Dunigan, a 36-year-old pharmacy technician, received Tuesday. She received the first stimulus payment in the spring without a hitch. But she wondered where her second one was after some of her co-workers received their payments over the weekend, and then her parents received theirs on Monday.

It was not clear why the IRS tool listed her payment status as not available. But she has used TaxSlayer, an online tax preparation service, to file her return for the past several years — and the firm deducted their tax preparation fees from her refund.

“I’ve been fortunate enough to work the entire pandemic,” said Dunigan, of Lake Somerset, Michigan. “I am not reliant on the money for bills. But I do know people who are reliant on it, and I cannot tell you how livid it makes me that people who are counting on the stimulus will have to wait.”

TaxSlayer did not immediately respond to messages seeking comment on Wednesday, but other tax preparation companies said they were working to get payments to customers where they could.

H&R Block said it was already passing along stimulus payments to customers through their bank accounts or via prepaid debit card. A spokesperson for TurboTax said the company was working with the IRS to help taxpayers receive their payments as soon as possible. And Jackson Hewitt suggested on its website that customers should consider taking the rebate on their 2020 taxes.