Are you trying to conceive? Are you tracking ovulation?
It takes the average couple 1-2 years to conceive once they start trying. If you are actively trying to conceive, and haven't after the 2 year mark, see a fertility specialist.
If you're asking why you haven't conceived while having unprotected sex for the last year, without trying to conceive. Well, it's called luck, or fertility issues. You're either not having sex on your fertile days, or you got lucky, or you have fertility issues.