No ovulation this month


I am currently on cycle day 20 and still no positive on my OPK!! For the past 6 months I have had a regular cycle, with ovulation occurring somewhat later than normal last month. I got my positive on day 18. Before that it has usually been closer to day 15/16. Is this normal? Should I keep testing? This month would have been our third <a href="">iui</a>. Maybe we will wait until next month? Thoughts??