Rara,
yes we also have that problem. My DH has to want to do it and when he knows he "has to" that just turns him off. My fertility specialist says it is best to do it 2 or 3 times a week when you are TTC, don't just concentrate on what should be the "right time" as the right time just puts too much pressure on. She has had male patients become impotent because of pressure to perform. That would be really awful so I have done what Satya suggested and just not told him when the right time was but just made sure that I made the atmosphere in the house as loving as possible around the right time eg. massages.
My DH suggested to me to get a calendar and mark on it the right week so he can look at the calendar and "know" rather than me feeling like I am putting him under pressure and him feeling under pressure.
It is not an unusual situation, so try not to get upset with him. I know that is hard as I have certainly got angry as well but after my FS told me the thing about impotency I don't get angry. I bet he is feeling worse about it than you are so maybe talk to him about what would help him.