Wondering how to ask for a raise at my job without stepping on toes, have asked once already.

It is almost as if my company is reluctant to give me a raise even though i lerform well, have great attendance, know all my customers by name and work hard to make sure my pharmacy is in federal and state code i also go to school for Software Engineering but i have 4 years of experience in being a tech 3 of which with the company and 1 with another there is also a union involved in the workplace because it is a retail supermarket store, but the union doesn't do F*** all and the District manager of the Pharmacy brushed me off saying i have an embargo with the Union (which is bogus according to the rep i spoke with but who knows) and the DM is sort of a Jack*** and your typical suit who doesnt care because he makes 6 figures and figures his workers arent his problem, but according to one of the salary calculators and various research i have done i am being severely under paid almost 13,000 a year at the maximum nationally and within the state by 3 dollars. Surprise surprise big retail chain under paying but, i want to get what i am worth and my boss and the other pharmacist know i am on that grind and that i am a very good and valuable employee but i do not know how to approach this a second time and in a assertive but polite way but also professional so he will learn that i am not a fool and know my stuff. Any advice is appreciated, his excuse was that i dont have the national cert, but he would give me a raise but not a big one from the 10.65$ i make hourly when most companies pay 13.62$ minimum in my state without the cert and with it probably between 15 to 21$ i have seen on the high end. But i like my boss and the coworkers i have but i have begun to interview elsewhere in the off event i can leave even though i have some reservations about doing so, and would like to see this situation to the end. Look forward to some good feedback.

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