New battery not charging after replacement

Hello,

I replaced the battery on my iPhone 6s and the installation went great, no problems etc etc. When I turned it on it has worked fine, and when I plug the phone in to charge it indicates that it is charging, however the battery isn't actually holding any charge. I have tried charging it without the metal plate that covers the battery socket inside the phone on (as some people have reported this to short the connection) and still no luck.

Are there any other common culprits for the issue, or is it a bad battery? I've tried multiple Lightning cords etc and so it's certainly an issue with the battery/charging mechanism.