I have a few different things to say, the first is as someone else mentioned, spicing it up. You wanna stay hard? Use a **** ring. An even better one than the normal ones is "The Screaming O". It has watch batteries so it's not one time use like the other ones. It has some little gel like bristles that rub against the clit and vibrates. It's used as a **** ring, but can also please the female. Make sure it is a lubed up a little so that it does please the woman.
There are various other *** toys you can use as well. Try lubricants that warm up when you blow on them. Try role playing and rather than taking off the outfits, keep them on to keep the mood going. There's also different types of furniture made specifically for ***, try getting one of those, the one that's coming to mind right now is the wedge style one.
Rather than just going for *** and stickin your **** in her, give her some oral, or while stickin your **** in her, rub her clit, breasts, kiss her, kiss her neck, etc.
Now here's my other thought. What is she doing the days you want to have *** and she doesn't? You say that she says she is tired, but is her wor/school/whatever schedule the same as other days you have ***. If so, it might be possible there may be another person involved and she is having *** with them and thus too tired to have it with you, because she already has. And the reason why she is able to *** many times with you, well besides being that is females and it is natural for them to have multiple orgasms, even within a very short amount of time, is that she hashn't gotten *** yet. I'm not saying that she is having *** with someone else for sure, but it CAN be a possibility, but hopefully it's not true.