My husband and I got pregnant right away with our first but we've been working on #2 for a year now - and not randomly, either! I've temped, tracked my chart with opk's and cm and had positive indicators that I was ovulating and that we were "doing it" at the right times. I am seeing a specialist now and we'll see where that goes, but I know that for some women baby #2 is just harder to conceive than baby #1.
If you don't know when you ovulate, take Shannon's suggestion and try some opk's (ClearBlue Easy and First Response are both good - and cheaper at Walmart than the drugstores). You might try speaking with your doctor, too, just to get checked out and make sure there is nothing amiss.
Look up "Secondary Infertility" in google or another search engine and read some of the articles that tell you things to try when you are having difficulty conceiving. I don't mean to imply in any way that you might have secondary infertility (which it what they call it when a couple has a child and then cannot conceive another), but there are many good articles that will guide you to things you can try on your own before consulting a doctor, such as reducing stress, limiting caffeine intake, reducing sugars, etc.
It's frustrating to try month after month with no results... I can remember being at the 5 month point and wondering what we were doing wrong. Just know that for some it takes a little longer, but it's always worth the wait! Hang in there! Shoot me a mesage if you ever just want to talk or would like some of the links to resources that I have accumulated.