Job Reclassification

Without giving out too much information. I submitted paperwork to have my job be reclassified as I am doing more work that also required a license. After getting the license my manger sent me to another department to be trained so I can start performing tasks that fell under the license. I am apart of a union and they dropped the ball on signing the paperwork in a timely manner so it was signed 2.5 months later. HR is now stating I will not be back paid for the date I started performing the work but from the date the paperwork was submitted by the union. Per contract language, “the effective date of the reclass will be the date of submission of the form but may be retroactive prior to submission date by mutual agreement”.

What are my options? Is this illegal? Union isn’t fighting for my 2 months of pay