The 2 most common reasons this year is you entered the wrong amount on line 30 for the Recovery Rebate Credit (3rd Stimulus payment) or line 28 for the Child Tax Credit. Those lines are just if you didn't already get the full amount or qualify for more. The IRS knows what they already sent you.
The difference is about 1400. $1,400 is the amount of the 3rd Stimulus payment. Did you already get the Stimulus payment they sent out last year starting March 2021? Do you have 1,400 on 1040 line 30? You might have tried to claim it again. Line 30 is only if you didn't get the full amount or qualify for more.
Did you get IRS letter 6475 saying what they sent you? If you didn't get the IRS letter 6475 saying what they sent you then check your bank statements and check your IRS account to see when and what they sent you.
https://www.irs.gov/payments/your-online-account
