It is definitely normal. Even healthy couples sometimes take time to get pregnant. We're just not at peak fertility for very long, and even when we are at peak fertility, it still isn't a guarantee that we'll get pregnant.
Also, many women get pregnant, but the pregnancy fails very early on, so early the woman never misses a period and never knew she was pregnant for that short time. That's usually due to chromosomal abnormalities in that developing baby that made it impossible for him to live and grow. It's not anyone's fault, it's just that there's a lot of chromosomes! There's a lot involved in making a baby, and it's easy for things to go wrong. It's kind of natures way of making it more likely that we'll have a healthy baby.
Ovulation calendars are accurate only to a certain extent. The typical ovulation pattern says that a woman should ovulate around day 14 of her cycle. The problem with that is that most women do not have a perfect 28 day cycle. That means they might not be ovulating like clockwork on day 14. If you use a calendar like
www.mymonthlycycles.com, then it will take your period's habits into consideration. That means if you have a cycle that is not 28 days, it will take that into consideration when deciding when you should be fertile.
Even that is not a sure thing though. The female cycle is a funny thing. We don't always ovulate when we "should." Some women even ovulate when they have their period.
Any time you are trying to get pregnant, make sure you are taking your vitamins, not drinking alcohol, and not smoking. All of that can affect a developing baby. Also make sure your ob-gyn knows you are trying. Sometimes ob-gyn can give couples some good tips that they wouldn't have thought of otherwise.