Is there a way to add shine to a chocolate coating after it has hardened?
What you're looking for is 'tempered' chocolate.   Tempered chocolate has that sheen and snap to it.  When properly heated and cooled, the cocoa butter in the chocolate formed the correct crystalline structure for this to occur.  When the process is uncontrolled, you get dull chocolate that doesn't snap quite as well. 

There's a few (1,2,3) different ways to do it.  But overall the easiest way - in general terms, is to heat chocolate to the correct temperature (check the links for correct temperatures for your type of chocoloate), stir in *pre-tempered chocolate (from the store or such) as it cools, and then slightly reheat.

Or, buy a tempering machine.

This should give you that gloss your after.



*Store bought chocolate is pretty much always tempered and the dull gray of the chips doesn't mean its not tempered - just scratched up.  You can use this to stir into your cooling chocolate at step 2.