Does ovulation

Does ovulation stay close to the same day every month or can it move and have a huge gap? The last two times I have gotten pregnant and then lost them have been cd 15 for ovulation but this very last time was actually cd 20. Now this month could it be closer to cd 20? Haven't gotten a + opks yet and tomorrow is cd 15, and it's no where close. But I think it happen earlier like cd 13... but I will yet til cd 20 as well. It just throws me off.. lol