Positive OPK, no pregnancy

Okay, I may be doing it wrong. I've always had sex on the day my opk turns positive. This is month 3 or 4 and I'm still not pregnant. Am I incorrectly timing my intercourse? Hubby has a physically demanding job so many evenings he's to tired to blink, let alone have sex, so we can't have sex every other day during my fertile period. I'm lucky of we have sex twice a week. With that being said.....when do I need to have sex?