Why cant i get pregnant?

Megan • Im 19. Ready for a baby of my own. I have 2 step children one is 2 ans one is 5. Ready to have a baby of my own though.
I have been trying to get pregnant for over a year. A year and 6 months exactly. And it just doesn't happen for me. I had sex everyday of ovulation 2 months ago. And still nothing. I've tried on my period, ovulation days and my So and I have sex every weekend and approx. 3 times a week. I just don't understand why its so hard to get pregnant. I have always wanted a family and I would have never thought I'd be trying this hard and this long to get pregnant. Why???