Pregnant problems!

Why am I having such a hard time getting pregnant? Granted we haven't been "trying" but we haven't been preventing it either! This past month we had sex 3 times in the week I was supposed to be fertile and....nothing! What is so wrong with me that some girls can get pregnant just thinking about sex, and others have to try and try? No one in my family has ever had fertility issues, in fact, most women in my family and very fertile, my cycle is normal, no history of any cancer or reproductive problems.... Help!!