Most common reasons are Erectile Dysfunction (not getting erection hard enough to penetrate) and Premature Ejaculation (discharges semen before getting in to proper act). Talk to him about your desire and suggest him to seek medical advise. You should be very cautious about not blaming or hurting him during this conversation. However, you put your desire in such a way that he has to pay attention to it. You have to address this issue very smartly but firmly.