I have two dependents but only got a refund for one. How can I fix this?One qualified due to age but other is 18 but was 17 at the time but is a full time student

You cannot get the child tax credit for a child older than 16.   Instead you can get the $500 credit for other dependents for the older one.  It goes by their age at the end of 2023.   If he was older than 16 at the end of 2023, you get the $500 credit.

https://ttlc.intuit.com/questions/1900923-what-is-the-child-tax-credit

https://ttlc.intuit.com/turbotax-support/en-us/help-article/taxation/additional-child-tax-credit/L6xFeMFEf_US_en_US?uid=lqnuygah

https://www.irs.gov/help/ita/does-my-childdependent-qualify-for-the-child-tax-credit-or-the-credit-for-other-dependents

Look at your 2023 Form 1040 to see the child-related credits you received

PREVIEW 1040

https://ttlc.intuit.com/questions/1901539-how-do-i-preview-my-turbotax-online-return-before-filing

Child Tax Credit line 19

Credit for Other Dependents line 19

Earned Income Credit line 27

Additional Child Tax Credit line 28

The $500 "Other dependent credit"  is "non-refundable". That is, it can only be used to reduce an actual tax liability.  No part of it is sent to you as a "refund".