Child Tax Credit Help

I have 3 children and according to Children and other Dependents Tax Credit It is around $6,000 (2,000 per kid). Trying to understand why my refund would be so low at $1,625 if I get $2000 per kid. Were low income and make less than $70,000 a year so surprised the refund amount isn't more.  I don't get  how it's $4,375 less.