What makes you think the OP is in a union?
In my circle, the answer would be “your friends”.
Keep badgering your boss. Eventually he/she will get the message. How many times have you asked? I find in situations like these boss’s go out of their way to work as slow as possible on the matter, but just keep bringing it up.
The only thing I want to add is that if you’ve repeatedly pressed your boss for more pay and nothing happens. And it seems like your boss doesn’t care about your requests. Then he/she probably doesn’t. And it’s a message to move on. You can stick around until something better turns up, but it’s time to start looking.
Complaining to your boss about your salary is counterproductive. Compensation is set by the market. It’s up to you to go out and find someone who is willing to pay you more. Once you get a firm offer, THEN you go to your boss.
Then the ball is in your bosses court, and your boss has to decide. Until then, your boss has no motive.
If your organization is the kind that discriminates against those who stay in touch with the market, you are surely being screwed every day you stay.