1400 is for the 3rd Stimulus payment for each person. It is on 1040 line 30. Line 30 is only if you didn't get the full amount or qualify for more.
If you claimed a missing stimulus payment on your return but the IRS took it off you have to ask the IRS. They think they already sent it to you. Maybe it went to an account you don't remember. Or you got a check or a card. It was probably easy to miss the debit card in the mail and think it was junk mail. They have to put a trace on it.
Double check your bank statements and Check your IRS account to see when and what they sent you (check both spouse's accounts if married)
If the IRS thinks they sent it to you but you didn't get it request a trace for a missing third Stimulus payment