These boards can be very frustrating. Since it is a community the activity on the board varies tremendously from day to day. I started to tell you how crazy my week has been and then realized, it doesn't matter, we weren't able to reach out to you as fast as you needed, that's what matters.
So my apologies, and my apologies to anyone else who feels they've been left in the lurch. To extend ourselves as support but not been there in a timely fashion just adds stress. Your note didn't say how long you've been trying or if you've had any fertility testing, so excuse me if my answer is a little off target.
As to your question. I don't think the problem is a timing problem. For most couples, if you have sex at least twice a week, by shear luck you're going to hit the right time for conception. I hate saying that to you, because timing of intercourse is such a concrete thing to latch on to, something that can be fixed if someone just tells us the right combination of days.
The honest answer is that there is a lot we do not understand about infertility. The percent of couples that we label as "unexplained infertility" may be as high as 50% in some studies. This doesn't mean there isn't a answer for those couples, this means medicine isn't perfect and we don't always have the explanations right now.
I think the other folks who have answered are saying the same thing, just in other words. Sometimes other modalities may help (see the reference about accupunture and the book another respondent referenced), maybe its time to speak to your healthcare giver about your attempts to date. I would encourage you to start looking at other issues besides the timing of intercourse at this point.
Hope this helps.