What Retards Size Of Follicle?

Hi, i have undergone infertility treatment by taking sitrodin injection. i have PCOS. Doctor adviced me to take ferpill tablets from day 2 and sitrodin inj on 5, 7 and 9th day. I have undergone scanning where follicle size didn t increase on 13th and 18th day. What is the reason for the follicle size didn t increase? Doctor even advised me to take ovacre tab from 13th day also but no increase in follicle size. I want to know the reason why follicle size didn t increase?