You did not earn enough to get the full amount of CTC. The child tax credit is used to reduce your tax liability to zero--and you have no tax liability. The amount of your income above $2500 is multiplied by 15% to calculate your Additional Child Tax Credit that will appear on Form 8812.
Look at your 2018 tax return to see the child-related /dependent credits you received:
Child tax credit line 12a
Additional Child Tax Credit line 17b (schedule 8812)
Credit for Other Dependents line 12a
Earned Income Credit (EIC) line 17a
Child and Dependent Care Credit line 12 (comes from Schedule 3 Form 2441)
The dependents you listed are on the first page of your Form 1040 below your address.
https://ttlc.intuit.com/questions/1900923-what-is-the-child-tax-credit
https://ttlc.intuit.com/questions/1900643-what-is-the-child-and-dependent-care-credit
https://ttlc.intuit.com/questions/1899157-what-are-the-qualifications-for-the-earned-income-credit-e...
https://www.irs.gov/credits-deductions/individuals/earned-income-tax-credit/use-the-eitc-assistant
https://ttlc.intuit.com/questions/4499708-what-is-the-500-credit-for-other-dependents-family-tax-cre...
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