My boyfriend had a daughter but we trying and i can't get pregnant what's wrong?

Desires pregnancy: The first step to see why you don't get pregnant is to see if your ovulating on time. You can do this with a home ovulation test kit or by measuring basal body temperature (it goes up 1/2 - 1 degree when you ovulate). If you're ovulating ok, then check your partner's sperm (semen analysis); final thing to check is tubes and uterine lining which are assessed with a hysterosalpingogram.