Don't rely on the 14th day rule would be my tip. We were trying for about 6 months and I always waited a while after my period finished, figuring that I would ovulate 14 days after the first day of my period like so many people say.
Last month I decided to try earlier in the month, in case I was ovulating early... and sure enough...., I WAS!
Also are you checking your cervical mucus? I didn't have that much but that can be a real give away.