What Causes Missed Periods Inspite Of Taking Deviry Tablets?

usually i dont have regular menstrual period....but this time i missed my period more than 20 days...then pregancy kit shows negative so we have done ultrasound scanning for pregancy it shows no pregancy and there is pco on both side.then i had deviry tablet for five days as prescribed by doctor..10 days completed from last tablet....still i dint get period.what could be the reason?