I think that it is possible not to ovulate every month. I began using an ovulation predictor kit a couple of months ago, and tested negative each time. I found it very stressful so I decided to take a month off, and actually got pregnant that month. I talked to my doctor about my anovulatory cycle before I found out that I was pregnant, and she told me that since I get my period every month (within 26-29 days) that it was extremely unlikely that I don't ovulate on a regular basis. She said that my anovulatory cycle could have been due to stress or a mild infection. Hope this helps!