Did one turn 17 in 2020? Or born in 2020? Only get it for under 17. A child born (even on Dec 31) is considered to live with you the whole year. So pick the Whole Year at the top of the drop down box.
If one of the children turned 17 before the end of 2020 that is why you cannot get the CTC for that child. Per 2020 tax law, you lose the CTC in the year that the child turns 17. Or...check to make sure you entered a Social Security number for the child, and see if you said the child lived with you for at least 7 months of 2020.
Delete the one that is not showing up correctly and re-enter the child as a dependent. Make sure you are saying the child lived with you the whole year, say NO when it asks if the child paid for over half his own support. Make sure you enter a SSN.