Gold Key Credit They claim I owe money for a hospital visit a year after everything was paid for by medicaid, Internet

Back in September of 2008 I took my daughter to the emergency room.  My ex-wife handles her insurance but was unemployed at the time and had applied for medicaid, which she got.  

The bill for a two hour emergency room visit was about $2500.  The insurance information was sent out and to the best of my knowledge everything was settled.  A year later I am contacted by Gold Key Credit telling me that I owe $509.  I asked why this amount was not paid for at the time everything else was paid for but no explanation was offered.

They acknowledged speaking with my ex wife but claimed that they never got the insurance information was never sent.  How is it possible that the other $2000 was paid for but not this mysterious $509?

I am now told that too much time has passed and medicaid will not pay the bill and they are unwilling to negotiate.

As far as I am concerned, they obviously received the information because the other bills were paid, but they screwed up somehow and now, a year later, they claim that I owe this money.  Why did it take them a year?

As far as I am concerned, they dropped the ball and now they are trying to pass the buck to me. I will dispute this charge against my credit and will not pay it!