How much is missing? A common reason is you claimed the Recovery Rebate Credit on line 30 or too much Child Tax Credit on line 28 but the IRS had already sent it to you.
Do you have an amount 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.
Double check your bank statements and check your IRS account to see what they sent you. Check both IRS accounts if married.
Your Online Account | Internal Revenue Service
