The laptop doesn't see a new battery after it being replaced

Hello!

I replaced a battery in my MBP 13" E2015 using the iFixit kit/battery. The process went surprisingly smoothly. I might've had to move the new battery around a bit one time, but I don't remember me bending or folding it too much.

With the power adapter connected, the laptop boots up into OS X; keyboard, trackpad, sound - everything seems to be working okay. Except the system doesn't see a battery :)

I replaced a battery because the old one went noticeably swollen. I never had "charging" problems with the old battery.

What would you suggest me to do? Later today, I will try to remove the back panel and unplug/plug the battery connector.

Thanks in advance!