Ovulated But No BBT Rise

I have been chartinng my bbt for 5 months now. The first three months I did not get a temperature rise. The fourth month we started TTC and I did get the rise.  This month (month 5) I've charted, checked CM and did an ovulation test which came back positive. Everything led me to believe that I ovulated. I had ovulation cramps, EWCM and watery CM. My temp had a .4 rise after ovulation but fell back down .4 degrees and has stated around that number. Is it possible that I ovulated but didn't get a temp rise? I read somewhere that some women don't get a temperature rise but it's rare and I would be surprised if I fell into that rare category. Anyone else in this situation?