the 2nd payment used the 2019 tax return to determine dependents. it did not use 2018 this time - is that the issue?
However, there is a 'settle up' section on the 2020 tax return,. You will be able to claim any missing stimulus you are eligible for there.
the way the stimulus REALLY works, is it is based on your 2020 income, filing status and dependents under 17 LESS the two payments received. if that number is positive, you get the credit for the remainder. if that number is negative, you do not owe it back