Alright, I'm new to the forum, have never owned a mac, or even a laptop for that matter.
I have however been doing my fair share of reading and there are a few other threads like this I've found. The problem is the battery won't attempt to charge unless it is partially drained to prevent overcharging. Since it's had a nearly full charge and has not been using the battery, the charge has not decreased or increased.
Run the battery down a decent amount and plug in the power adapter and it should charge it all the way up. If not you can always calibrate your battery by draining the power completely and giving it a fresh charge.
at least I hope I'm partially right 🙂