feeling down....

I have three children, all of whom were conceived without a lot of effort, now that my husband and I are trying for our fourth, it seems as if it will never happen smh...we've only been trying since February but it makes me so sad every time it doesn't happen. I've been to the doctor and I've had ultrasounds and everything has come back normal ..I don't know what could be wrong or why we cant get pregnant. My husband says it'll happen when God wants it too, and I have faith but, every time someone gets pregnant I get even sadder..what could be wrong?