What Causes TB Of Intestines While On Myrin?

My wife was diagnosed with TB of sacral region. She undergone surgery and has been on Myrin-p (later on to Myrin) and Vita-6 for the past 14 months. Now ultrasound has detected TB in intestines. I wonder if this existed earlier and was not diagnosed OR is a new development. In the later case, how come Myrin-P/ Myrin has not been effective. What are prognosis in such a case?