There a dozens of reasons someone might not get pregnant. You could have fertility issues, your partner could have issues, you could be having sex on non-fertile days, you might not have been trying too long (it can take up to 2 years to naturally conceive a child). Your best bet would be to see a doctor and figure out if anything is wrong. If there's not, they can give you advice on the best ways to conceive.
did you check with your gynecologist