I havent had to amend any returns that I didnt prepare the original, so Im not sure what advice to give.

If you created an original exactly as filed, and marked it for Efiling, you would have gotten an original 8879.

If you then opened the 1040X and marked the 2020 box, then went back and made the changes youre amending for, and marked the amended EF box on the Fed info worksheet, it "should" have updated the figures on the 8879.

If thats not happening, Im not sure where to go with it, sorry.