How do I have a former employer reverse a "do not rehire" designation following my own resignation?

Resigned 2/17 due to another job opportunity having left on good terms.
New opportunity fell through and 2 months later I reached out to former employers hiring hall to place myself on the union list for a temporary job assignment.
I was extended an opportunity but a few days later was notified I had been designated as an "In-eligible for rehire" candidate with no explanation.
I was also contacted by an employment agency wishing to place me on a long duration contract with my former employer but was not able to proceed due to the "In-eligible for rehire" designation.
I contacted HR and submitted an appeal requesting a review of eligibility. Review committee did not reverse decision but have never provided the specific details as why I was categorized as In-eligible for rehire.
I would like to examine potential opportunities with my former employer, within my area of expertise however cannot do so while this designation remains on my employee file.
What steps or measures must I take in order to have this addressed in as expeditious a manner as possible? Any assistance or guidance would be greatly appreciated.