2800 is part of the 3rd Stimulus payment on line 30. 4200 would be the amount for 3 people (1400x3). Like married with 1 dependent. Did you add a dependent on your return? Like have a baby?
The IRS records show they already sent you the 2800 3rd Stimulus payment so you only qualify for another 1400. Line 30 is only if you didn't get the full amount or qualify for more.
The 3rd Stimulus payment started going out in March 2021 and is for 1,400 for each person listed on your tax return, no age limit. The IRS used your 2020 (or 2019) return to send the checks out fast. If your 2021 return changes and you qualify for more you can claim the extra as the Recovery Rebate Credit on 1040 line 30. But line 30 is only if you didn't get the full amount or qualify for more. Not the amount you already got. The IRS also sent out letter 6475 to tell you how much they sent you . If you are married then each spouse should get a letter for half.
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
