It cane either be the guy's problem by being impotent either from medical history or stress.Or it can be the woman's fault either through complications of the womb or Fallopian tubes or being sterile.That's why couples who find it hard to conceive have to go through so many tests - it's to see why and who's at fault.
And this is exactly why we need sex education. Women don't produce sperm, the produce the eggs.
Possibly because girls don't have sperm. There are numerous reason as to why some people aren't able to conceive. It could be something that's wrong with her, or something that's wrong with her partner. I have 4 friends that are unable to conceive and 2 cousins and none of them have the same problem.