Claims not paid

In December 2015 -I was admitted to a hospital that Bankmed had referred me to which is Medforum.When I went to a check up in Feb to the same doctor that had treated me in December ,he had to admit me again into the same hospital that he treats in which is Med forum..I was admitted again in February 2016 ,an authorization was already done by the hospital and now when I am getting my claims statement, they are only paying 80% of the hospital stay as I did not use 'their' service provider. It is not my fault if Discovery and Bankmed decided to merge.I sent so many emails and not getting any joy.They are still declining my requests after I had explained to them telephonically and by email.This is unfair treatment.Am I supposed to jumo from doctor to doctor because they decided to merge?The hospitals that they are using now I have never been to any of those doctors,now because of the merge I have to change my doctors