You really HAVE to chart. It sounds like a weird/time-consuming thing, but its actually very, very easy & you'll end up having fun with it. it is teaching me a LOT. OPK's are apparently just a "2nd level sign". A 1st level sign is your temps. Try it at fertilityfriend.com or any others! You won't regret it (took me 10 months TTC to do it!)
Even tho all of my OPKs were negative for 9 months straight, blood tests did confirm ovulation, so you may have just missed it. Even now, w/ClearBlue Easy smileys, I only show a +OPK on 1 day, so it takes a lot for LH to register for me...
Good luck!