Why am I receiving less than what is calculated for the advance child tax credit? I have five children, ages 14, 11, 7, 6, and 3 by year's end. What can I do to correct?

From my own calculations as well as double-checking with the child tax credit calculators, I am receiving $50 less per monthly payment. It seems that my payment is calculating all of my children to be older than 6. How can I fix this?