I'm trying to use my comiXology eGift Card balance to Subscribe or Pre-Order but it's not working!
You can, but those types of purchases will only take place when the subscribed or pre-ordered comic book is officially released. So, when you Checkout with those items, you won't be charged. It's entirely possible that you may make another purchase and use up all the credit before those Subscriptions or Pre-Orders come out. If that happens your credit card would be charged for those items, and not your empty eGift Card credits. Just a heads up!