If you are referring to the Third Economic Payment, that is based on your 2021 return (which we are working on now) so your 2020 return would have no bearing on this year's credit.
On the 2021 return, your credit is calculated on your 2021 information reported on the 2021 return, and the payments you received are subtracted. If you are eligible for more than you already received, you will see that amount as the Recovery Rebate Credit.
If you are referring to the First and/or Second Economic Payment(s), any portion still due needs to be claimed by filing an amended 2020 tax Year return.
IRS information about economic payments
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