Employees who never received a pay increase when they should have

I recently found out from my union rep that last year when I became red jacket certified I was due a .25 increase. He checked my compensation and it did not occur. He immediately sent emails as well as my line manager sent emails to HR. This was three pay periods ago. Im owed about 1500 hours of back pay for that money as well as im still nor being paid correctly. To make matters worse we have a new hire. He is still in 90 day probation period. Out of the blue he tells me what they are paying him. Ugh he is being paid .25 more than I. It's maddening because I work harder am more experienced and am teaching him. I like the kid but am humiliated because of how much I do for my lead, line manager and entire store. I hate to file a grievance. My manager said that HR hasn't responded to her yet. UFCW 876 Detroit