Hi there: Yes, it's possible that you are going to ovulate without seeing a pre-ovulatory dip and yes, it's possible to have cycles with varying patterns. You might find you O on cd 14 95% of the time, but then have a moving target date for that other 5%. Or you might find you O 1/2 of the time on cd 14 and the other 1/2 on cd 20. There is no end to the variations! Your pattern might be that you have no set pattern

.
What should stay roughly the same, though, is the length of your luteal phase. So, if you O on cd 14 and get AF on cd 29, then your LP is 14 days. This can vary by a day either way (a LP of 13-15 days, in other words), but ideally it shouldn't vary by more than that. So if you O on cd 20 one month, you should get AF on cd 35 that month (assuming a 14 day LP).
I don't think you should assume you are NOT going to ovulate this month. It might be the case (every now and then even the most regular ovulater has an "off" month), but I would not stop BMSing because one of the most frustrating things is to give up too soon and then miss O!
So, short answer: keep charting and OPKing and most importantly, keep BMSing .... and I bet you'll see your rise soon (but maybe not the dip beforehand!).
Good luck and keep us posted++++++++
Maura