Child support for my daughter in South Carolina

This goes back a couple of years. They say I am like over $3,000 behind. I have sent to them a fax and mailed to them the statement from Western Union showing that I in fact had sent over $4500. They asked my ex into the office and asked her if this was for support. She said some of it was. They said it had to be all or nothing. So her response was than nothing. I tried to say to family court why would I send her extra money when I had been behind. That it was in fact for child support. How can she say yes half can go toward child support and than they don't accept any of it towards it. She admitted that she had recieved it but wouldn't say it was for child support. So I don't know what I can do. I have paid it.