Taking duphaston is not good?

Since when i was 13y.o my period is regular. I have monthly period every month.last year july 2015 at the age of 25y.o. i got my preterm labour at 6 mos only.... January 2016 my ob prescribed me to take duphaston every day 16 of my monthly period for ten days...... I'll stop taking duphaston last month april 2016...i expected my period this may 4 or 6, 2016 but until now no sign of period comes...... I use pt yesterdsy it turns out negative....... May i know the reason why.... Thanks