Passport Denied

My kids dad is on child support and only owes 1400$ back pay for child support. The federal rules says you have to be under 2500$ behind on child support or your name will be added to a certified list of not being able to receive passport . So if he only owe 1400 why is he still being denied. Anyone that have knowledge please help what should he do????they have us 90 days to get resolved