Some families have not received their September Child Tax Credit payments, which go out on the 15th of every month until the end of the year.
Parents are eligible to receive $300 per month for each child under 6 years old and $250 for children between 6 and 17.
"We are aware of instances where some individuals have not yet received their September payments, although they received payments in July and August," the IRS said in a statement on its website.
The IRS added that it's looking into why the payments have not been delivered.
The agency said the third round of payments went out to approximately 35 million families.
The IRS says the value of those payments is about $15 billion.
The monthly payments are scheduled to end at the end of the year. However, CNBC reports that House Democrats have proposed extending the payments through 2025.