Credits using the 2019 income on the 2020 return

[dritz]

The clients returns have been efiled using the 2020 income credits.  Using the transmitted return and the 2019 income for credits the refund is $2800 greater.  However when I engage the 1040X from the original return, add the 8812 checked for using the 2019 income the refund is only $400.  What am I doing wrong?

[Just-Lisa-Now-]

the 1040X wont be ready for use until late March.

[dritz]

Other forms are not approved but compute correctly.  Why would the X not be able to do the same?