Job: Department Manager, Retail Store (United States)

I have been working for Walmart for more than 5 years now. I just took dept. manager last April mainly for the money. I got a $0.40 an hour raise. I am now making $10.00 and hour and most of my workers are making the same or more than me. Their is one worker in my lab that started at Walmart 4 months after me in a different department. She transfered to my department in April last year. She is now making more than I am as the manager. Another worker has been with the company for 3 years (compared to my 5) and she got a cost of living raise that I didn't get. So now she and I make the exact same amount. How is it that someone can work their tail off and get no where when the others don't know or do half as much as I do but are getting all of the raises?