I can't see that there's anyting you can do about it.
Unless there's a statutory pay scale (as in teaching) or an employer agrees to negotiate with a union (or staff association) for a common pay package, each contract between an employer and an employee is entirely separate.
As long as there's no breach of discrimination laws (or of minimum wage regulations), an employer is perfectly entitled to pay different employees at different rates. If you've tried, without success, to ask for a pay rise, all you can do is to look for another employer who pays more.
Chris