Replaced Battery but it will not charge

Hi -

I just replaced the battery with a new one from ifixit.com. I turned it on and everything works. I immediately plugged it in to get a full charge on the new battery but it doesn't seem to charge.The lightning bolt comes up but then it goes to the plug icon which usually means fully charged. I unplug it to see if it is and its stuck at half way. Do I have a defective battery or did I do something wrong? I left int plugged in for 2 hours and it hasn't passed the middle charge yet.