When you request a leave, put the request in writing, explaining the reason for leave and how long a leave you need. Keep copies of everything you send to and receive from your employer, as well as copies of doctor's notes and any other medical documentation.
If you are denied leave, you may want to file a grievance. If you are a union member, you may be able to file a formal grievance through the union. Try to get a shop steward or other union official to help you work through the grievance process. Some employers have policies for handling a dispute regarding leaves. You may be able to resolve the dispute internally with your employer. Find out what your employer's policies are by looking in your employee manual or other sources of personnel policies. If your company has a human resources department, they may be able to help.
However, even if you file a grievance with your employer, the deadlines to file in court or with an administrative agency still apply, so be sure not to miss them.