sometimes you can have scant fertile mucus and the only way to know it's there is to check internally, directly around your cervix. that might make some people queasy to do that--but it's your own body, and it shouldn't be a big deal...
sometimes people ovulate really really early, you may be OV towards the end of your period and have the stretchy stuff mixed in w/the menstrual blood and not even know it, believe it or not.
and it is possible to stop OV from time to time, but i'm not sure if you'd still be getting your period from month to month. i stopped OV when i went off the pill last year, and didn't have my period for 5 months.
so when my periods returned (thank GOD) i knew i was OV again and i got to bd-ing RIGHT AWAY!!!
you are supposed to temp at the same time every day in order to keep a consistent chart, but of course not everyone can do that. just do the best you can. honestly, look at the fertlityfriend website, and check out the toni weschler book b/c they both provide amazing advice about all of these things.
i hope this helps. good luck.