prior authorization from oxford prior to UHC takeover

Hello,

We had an Oxford Medicare patient who was admitted to hospital end of December 2011 after we received prior authorization from Oxford for the admission. On January 1 Oxford's admin was taken over by United Healthcare. Our patient continued to received services by our doctor in the hospital for one full week into 2012. Patient had been automatically transferred over to United Healthcare Medicare Advantage on 1/1/12.
United healthcare is refusing to pay for the January 2012 portion of the bill stating that patient did not have prior authorization. Yet their documentation regarding the transition clearly states that their 'notification of inpatient admissions wil follow current Oxford guidelines.'
Appeals are failing, they keep repeating that we did not have prior auth for the admission.
Anybody else having this problem? Have you been able to resolve it?
thanks so much

Howard