A man’s fertility rate generally relies on the quantity and quality of his sperm. If the amount of sperm a man ejaculates is low or if the sperm is of poor quality, it will be difficult and sometimes impossible for him to impregnate a female.
According to Prof. Oladapo Ashiru, a male infertility is usually caused by problems that affect either sperm production in the testes or sperm transport.
See 10 of such causative factors of low sperm count in men.