Day 18 and still have not ovulated, what could be the cause of it?

Happens sometimes: In a non-pregnant woman with regular periods, some causes of a late ovulation or a late period include: excessive weight loss or weight gain, increased exercising, emotional stress, having an illness, traveling, medicines (like birth control pills), hormonal imbalances, etc… Seeing a doctor is a good idea if symptoms are persistent, recurring, or worrisome... such as no periods but not pregnant.