The dependent that did not get a stimulus payment would not be qualified if they are over the age of 17.This could be the reason you are not getting additional stimulus money with the Recovery Rebate Credit. In order for a dependent to be eligible, the dependent must be your qualifying child under the age of 17. Please keep in mind that although they may be dependents on your tax return, they are not automatically qualified for a stimulus payment.
If the information is not being picked up for your second child, it is possible that TurboTax is not recognizing the social security number, or that a question in the interview was not answered correctly. Try deleting the children in TurboTax and then going back and entering their information again. Make sure that all fields are entered correctly.
To qualify for the stimulus these conditions must be met:
A qualifying child, under the age of 17 for the 2019 tax year, who has not provided more than half of their own expenses and lived with the taxpayer claiming them as a dependent for more than six months is eligible for a stimulus payment.