What Causes Late Periods After Pregnancy?

HI Doctor, My name is ahmed and my wife has delivered a baby 4 months now, it was a normal delivery, and we were in physical contact as well, but my wife never complains about pain or vomiting or anythign which are the signs of new pregnancy and even though we consulted to one of the Gynecologist here but she said it is normal and periods will return but till now she is didnt get the periods and she does the breastfeeding both at the day time and night time, can you please tell us what is the problem we are worried. Thanks in Advance