Usually whenever I have problem, they fix it for me quickly. Only one time, I have problem related to the special promotion and items unable to deliver, it took me few email to resolve this. They ask me to call too, but I insist to solve this through email as the evidence of the problem is very obvious and I don't have much time to spend over the phone.
If the problem is reproducible and there is evidence of the problem shows up in your account. You can just point it out and ask them to look at it. In your case, I believe there must be a pending entry taking out your entire gift card balance under your gift card transaction history. Base on this information, their tech people should be able to figure out what went wrong from their transaction log.