The law does not require that employers pay all similarly situated employees the same compensation typically.
Only where the employer engages in prohibited discrimination or retaliation would courts normally get involved and order retroactive pay to be paid.
If it does not appear the employer is engaging in prohibited discrimination and there is no contract of employment, there is normally no method for compelling the employer to increase the employee's compensation.
The threat of such litigation from a local attorney could motivate the employer to pay, but it does not appear you have a valid legal claim against the employer.
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