Hi James,
To be honest, it seems to me that Google has some sort of quality control issues with their replacement parts (which is what iFixit sells). It's been mostly concentrated with their screens, but sorry to say it wouldn't surprise me to find that those issues are also plaguing batteries as well.
You should definitely get in touch with iFixit's Store personnel and talk to them about the possibility of a replacement. Bad parts do happen and my experience with them has always been positive. Best way is usually to email them at support@ifixit.com.