If one of the dependents was older than 16 at the end of 2020 that is why you did not receive stimulus for that dependent. If all of them are under the age of 17, use the recovery rebate credit.
The stimulus check is an advance on a credit you can receive on your 2020 tax return. If something went wrong or you did not get the stimulus check in 2020, you can get it when you file your 2020 return in early 2021
The Recovery Rebate Credit will be found in the Federal Review section, If you are eligible it will end up on line 30 of your 2020 Form 1040.
https://www.irs.gov/newsroom/economic-impact-payment-information-center-topic-j-reconciling-on-your-...
https://www.irs.gov/coronavirus/coronavirus-tax-relief-and-economic-impact-payments-for-individuals-...
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