Vibrancy of chocolate sauce
There is nothing wrong with your recipe. This is not only typical, it can't be changed. 

The human sense of smell is triggered by molecules floating in the air we breathe. When you make a chocolate sauce, some of its molecules are volatile - that is, not very attached to the sauce, ready to leave the surface and float off into the air like a helium balloon when a gust of wind tears its string - and others (most of them) are not, they stay put where they are. 

The volatile molecules get into the air, we breathe them in and so we smell the chocolate. But after some time, all the ones which were volatile to begin with have floated away, and are not any longer there. 

The other thing which is happening are probably some chemical reactions. I am not sure of the details, but with something as complex as chocolate, there has to be a bit of degradation going on. With chocolate being so stable over long periods of time, I'd say this contributes much less to the effect, but I am adding it here for thoroughness. 

So there is nothing to be done. If you try to somehow "anchor" the smell molecules to the chocolate, they won't go away, but they won't be available for being smelled either. Either way, you cannot get the smell to linger. If you want sauce which smells fresh, you have to make it fresh.