I have the same issue, but since the Marketplace indicated that the purchase failed, I purchased again and it worked the second time… except I was charged twice.
As I didn’t find out immediately, I have 3 DLC paid twice (and available) and 1 paid once but not available in game.
I have tried to issued 4 requests on the web site but you have to confirm the request through mail afterwards : I haven’t received the confirmation emails 30 minutes after submitting the requests…