Stress is one of the biggest factors when trying to have a baby... and it can mess your cycles up. It took us 13 months to conceive our last pregnancy. The MOMENT I said, "Enough is enough, I'm focusing on other areas of my life" is the moment we finally conceived. And I seriously did not try to have a baby. We just had "fun" and I focused on weight loss.
So as hard as it is to try to not stress, it can definitely have an impact when you're trying. Also, be consistent- keep having sex, even if your period isn't showing up. It could be that you're ovulating late.
Another great thing would be to learn when you ovulate. There are many ways and methods out there to determine when you are fertile.