Have you entered all your income? The child tax credit is a non refundable credit meaning you must have $8000 of tax liability from taxable income after subtracting the standard or itemized deductions in order to get it. If you don't have enough income to have an $8000 tax liability, the amount of child tax credit will be limited.
There is an "additional child tax credit" that you can get even if you don't have enough tax liability to get the child tax credit but it is only up to $1400.
