State Collection Services, Inc lost $400 payment Madison Wisconsin

I set up a payment plan with State collection service inc, in 2012 to pay off medical bills.  They told me I had to make a larger payment in order for them to set up smaller month to month payments.  I sent them $400.  For 3 months they debited smaller payments from my account to pay off the remaining balance.  The week of the last payment I received a letter notifying me that I still owed a larger amount.  When I called it was the $400 that I paid in august.  They had no record of it and I could not speak to a supervisor.  He would not return phone calls or anythying.  Now 6/8 months later I got another letter saying they are taking me to court.  I have not heard from them since oct/nov of 2012.  My $400 payment is still missing in their system even after I provided bank statments showing I paid it.