Which stimulus payments are you referring to, there were three?
The two stimulus payments that you would report on the 2020 federal tax return?
Or the third stimulus payment being distributed by the IRS and is not reported on a tax return?
If the third stimulus payment go to the this IRS website for information - https://www.irs.gov/coronavirus/get-my-payment
The round 1 and round 2 stimulus payments are reconciled on your 2020 tax return. Any amount of stimulus payment will be reported in a letter number 1444 or 1444-B sent to your from the IRS. If the IRS believes they issued a payment, you can't claim a rebate on your 2020 return because the IRS will automatically remove it. You have to trace the missing payment instead.
See question F3 on this page.
The round 3 stimulus will be reconciled on your 2021 tax return. Although we don't have the 2021 instructions yet, it is likely the procedure will be the same. If the IRS issued a stimulus payment, they would send you a letter number 1444-C. If the IRS sent a payment but you didn't receive it, you will have to initiate a trace.
Look at the return you already filed ... did you report the missing stimulus payments on line 30 ? If you did then you do not need to amend the return. If the IRS processes the return and removes the credit then you need to initiate a trace of the missing money...