Why do some couples take longer to achieve pregnancy?

Q: Some couples are able to achieve pregnancy within half a month of regular sex, while others get it later. It is recommended that they see a doctor after 12 months of regular unprotected sex, why is that so? Isn't one time proper sex enough to achieve pregnancy?

A:That's because statistically, 80-85% of couples, after unprotected coitus can have a pregnancy within one year. The remaining people are then investigated.