Why are we not getting the child tax credit?

My spouse and I are self employee. He is an Uber driver and I am a braider. We have 4 children ( 2, 6, 10, and 18 years old). When we entered our income and expenses , we owe $500 to Federal but are getting a refund from our State. What I don't understand , is why we owe if taxable income is zero?

Also why are we only getting $1249 in Child and other dependent tax credits?

How can we get the full credit for our children?

 

Thank you for you help