system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 Why is it normal for a healthy couple having regular sex to take up to a year to conceive?

Sex during a fertile period seems to almost guarantee offspring in many other mammals, so why does it frequently take healthy humans a number of attempts? If all is well with both partners and sex is timed perfectly, why isn’t conception guaranteed?
gpt
Humans have concealed ovulation.  Why?  It lead to higher rates of survival, that's why evolution does things.  The popular theory is that it increased the bond between partners, causing fathers to hang around and help with the kids more.

To say "timed perfectly" isn't really how it works.  These biologic processes are highly variable, nothing is ever guaranteed.  Diet, stress, sleep, and a zillion other factors contribute to fertility.