why am i not pregnant yet ?

I have regular periods of 27/28 day cycle. I had been to my gynac for pre mentrual bleeding (occurs around 20th day after we planned for a baby) on the 13th day of cycle and found that my ovulation is around 14/15th day. The doc said there is nothing wrong with the scans, the pre mentrual bleeding will come down on its own. the next cycle i did not have any pre menstrual bleeding. We have been trying for a baby for 3 - 4months now. we have sex on 13, 14, 15, 16 day of my cycle. Yet my periods come on their usual time. this time though it was late by a day and for two days its was hardly noticeable, but then on the 3rd day it was a normal bleeding. Why am I not getting pregnant inspite of timing it properly. My doc says give some more time..