I don't know, and you have not told us what they said, but there are a couple of forum members with long stories and multiple detailed threads on how this happened to them and how they are combating it. The 2 chief causes of this sort of IRS ruling (again this is 4th hand from my reading of those threads) seem to be AMT issues and Schedule C usage. In one case the car was not the Schedule C partial business usage vehicle, but was treated as if it was..... I suggest you search for and read those threads in detail, and then start your, unfortunately lengthy, appeal process... might be a good time to bring in a tax advisor....
it is my understanding, again I am NOT highly knowledgeable on this subject, that the wording of the tax credit law makes it clear that the credit should be after all AMT calculations, are taken into account, but that some IRS staffers are not interpreting it that way, citing some other wording in some other section of the tax code....
if that isn't murky enough, the schedule C issues threads read like sections of Catch-22, It seems that if your Volt is business owned, or partially amortized as a business expense, or even not depreciated, but only charged of as a per mile deduction, that some interpret one of these conditions as changing the tax credit value.... how I have no idea.
good luck