Why am I not pregnant yet?

I am 21 and I already have a 4 year old daughter. My husband and I have been trying to get pregnant for 3 months now. I track my ovulation using an online ovulation tracker. And still nothing. I guess I am most frustrated because my daughter was a complete accident I was 17 a junior in high school and it was almost my first time having sex. And boom I was pregnant. If anyone can suggest anything that would be awesome!