Fertility help, getting discouraged.

Hey ladies, I've been trying to get pregnant for about 4 months now and it's not happening. My periods are perfectly regular, their synced up with this app lol and I've been following when it says I'm fertile. It only took me one month to get pregnant with my first and I know my boyfriend is perfectly fertile, I just don't understand what's going on. So any advice is helpful and welcomed. Doctor did some blood work, checked some hormone levels and said I'm fine...I'm not getting it.