Print out the 2019 and 2020 returns and compare them line by line. It will be difficult since the forms were redesigned between the two years so the line numbers will not match up. Then wherever you notice a discrepancy, review that topic.
It can even be something as simple as a typo, such as numbers with digits transposed, a decimal in the wrong place, a comma in the wrong place, etc.
It's also possible that your income did drop, but only you can know that.
