past due child support and tax refund question?

so my x is 1000.00 behind on child support in texas , she said they took it out of her tax check I called the ag office and they said the irs can hold it for up to 6 months , not im confused because she has a child from another relationship and he was behind on support and she got the check shorty after getting her personal refund check. irs holding child support for 6 months , doesn't make sense to me ?