Why recovery rebate credit credit reduced

If the IRS reduced your Recovery Rebate Credit, their records show you received more in Stimulus Payments than you reported having gotten. 

If the TurboTax program reduced the credit, it was an adjustment made after you reported the amount you already received as a stimulus payment. 

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 (both accounts if married) to see what they sent you.  Check both IRS accounts if married.

Your Online Account | Internal Revenue Service

If the IRS thinks they sent it to you but you didn't get it request a trace for a missing third Stimulus payment

https://www.irs.gov/newsroom/questions-and-answers-about-the-third-economic-impact-payment-topic-j-payment-issued-but-lost-stolen-destroyed-or-not-receivedYour Online Account | Internal Revenue Service