So you pretty much only represent the worthless employee's at the expense of the good employee's, this is what I suspected.
And why I don't like unions.
You cannot say you are representing the good employee's because you are forcing them to carry the dead weight of the bad ones. You should be helping the company do proper documentation and follow up on it. Other wise the mediocre employee will see what the bad employee is getting away with and become a bad employee.
You can still defend the useless, as best you can, but if he has no defense your hands are tied.