What Causes Delay In Periods Even After Taking Deviry?

hello doctor
my wife name is sanjana age is 28 and last delivery time is 7 years she is suffering from pcos for last two years. and now the problem is her period was due on 6 dec.and now today is 20 dec period was not come she go to the doctor she said take deviry 10 mg for three days 2 tablet in one day.she start taking tablet from 14 dec.and we check pregnancy urine test that was negative .she take last tablet on 17 dec. period not come .please give the answer.