How much did you have on line 30?
How many dependents do you have?
Did you get a IRS letter 6475 saying they sent you 2800? Each spouse gets a letter for half. So I think you had 2 dependents and got 5600 (1400x4). But only entered one letter for 2800. So it gave you 2800 on line 30. Line 30 is only if you didn't get the full amount or qualify for more.
The IRS records show they already sent you the 3rd Stimulus payment. But you you tried to get it again. Double check your bank statements and Check your IRS account (both accounts if married)
Your Online Account | Internal Revenue Service
