I dont understand...

So me and my boyfriend have been trying for a couple of months now.. How is it possible for a women to not get pregnant even after having unprotected intercourse all thru out the month? Even on my fertile days at least three to four times during that time always a couple days before my fertile days? I didnt have my last two pregnancies planned let alone want any now that I have em i wouldnt change it for the world but i obviously got pregnant easily... Why is it so hard now?