That is one of the generic reasons why they adjusted the Recovery Rebate Credit on your return. Do you have an amount on 1040 line 30? See this IRS article
https://www.irs.gov/newsroom/irs-letters-explain-why-some-2020-recovery-rebate-credits-are-different...
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.
