Having problems at work?

I work at the front end of a grocery store as a bagger. According to all the supervisors and managers, I am one of the best they have. Four supervisors had to write a letter of recognition for one employee. All four chose me out of about forty others. So I must be doing something right, Right? Unfortunately, they will not put me on the cash register full time despite the fact I am one of the fastest. I asked them but they said I was the best bagger/cart pusher they had. It upsets me that someone who was there one day longer than me is already a supervisor and several others have moved up to cashier as well. I hate pushing carts all night long and they know this. One of the things all the letters say is how I never complain; however, everyone who complains seems to get what they want. I have a feeling I will never move up, get a raise, or an increase in hours (avg. 28 per week). How can I fix these problems? Also how and when can you switch departments?