Time to meet with the boss. Let them know of your interest in moving up and ask what you need to work on to be considered for the next position. Ask that your salary be reviewed also. Don't mention that you know others get paid more than you do. That's none of your business and it won't help you get a raise. No need to get upset. You are probably doing such a great job where you are, that you make their lives easier. They either want you to stay where you are because of that -- or they haven't realized that you are interested in other positions. Have you been applying for them?