On the basis of what you have posted, there is no illegal discrimination. No Federal or MA law requires that you EVER get a raise unless minimum wage is raised by the appropriate legislative body AND you are working below the new minimum. Nor is there any Federal or MA law that says if one employee on final warning gets a raise all employees on final warning must get one.
Unless you have some valid and supportable evidence that you were not given a raise when other employees did BECAUSE OF your membership in a group protected by law (race, religion, national origin etc.), your only option is to improve your performance to the point where your employer is willing to give you an increase. Please note that it is not enough that the other employees BE in a different category than you are - the adverse treatment must be BECAUSE you are in a different category than they are.