Most motorcycle batteries only last 2 - 3 years. If the bike was sitting for any length of time with the battery connected it will run flat.
So, if the battery is 2 - 3 years old or more, and been sitting for months, then there's a good chance it's dead and no amount of charging will bring it back to life.
Sure, you might get a 12v reading on a multimeter, but it will have lost it's CCA power which is what you need when starting a motorcycle.
