i also find it depends on the weather. Have had to up my chocolate ratio massively with the heat we have been having lately. Also, to avoid grainy ganache, I find using a metal whisk does the trick. Not sure why it works so much better, perhaps because the whisk heats up too and melts the chocolate as it contacts it? (I make my ganache on the stove top =)) and dont be scared to reheat ganache.. its one of the beauties of it.. it wont hurt it to reheat it.
12/5/12 at 4:13pm