When you ask for one you should request it in writing. Nonetheless, a union's bargaining efforts must still be in good faith and not be the result of arbitrary or discriminatory reasons. The specific procedures and requirements pertaining to membership involvement in contract proposals and ratification are described in each union's constitution. If a union fails to follow their constitution, it can be found to have breached its duty of fair representation. In others, members depend entirely upon paid staff.
Some unions are more democratic or militant than others.
You will get more from your union if you're informed, active and demand that to which you're entitled than if you're uninformed and complacent. Tips for handling grievances, discipline, etc. Note down what is said to you and what you say. Your representative cannot read minds. He or she depends upon your ability to communicate.