Ok. How much is on line 30? How much did you actually get in 2020 and 2021 for the first 2 payments? How many people are listed on your return? Married + children under 17.
They might have increased your income so you went over the max to qualify for the Recovery rebate credit. Or they took off one or more of your dependents.
You might have reported the Stimulus payments you got in the wrong boxes so it gave you the wrong credit on line 30. Or reported the wrong amounts.
