Can you try plugging in the old battery? If the old battery functions normally (outside of whatever reason you replaced it in the first place), then that would be a pretty good indicator that your new battery is defective. To the best of my knowledge, iFixit does their best to find and sell quality parts, but there's always a chance you'll get a bad apple in a barrel. My experience with their customer service has been exemplary so I'd recommend contacting them if you have reason to think the new battery is bad.
Best way to get hold of them is via email at support@ifixit.com.