Like how does management let the associate know that they want to promote them, are they straight forward or are their signs? What happened when you were promoted?
Associates don't get promoted out of the blue. When there is an opening, there is a bid open that is through the union. All available department head positions in your district are usually posted somewhere in your store.
If you're talking about front end, just because some part time college kid wears the red vest does not mean they are on a higher pay scale. Actual front end supervisor is the same as the department head of other departments. But for some reason, front end can just hand over supervisor posts to anybody who is smart enough to let others know when they have breaks.
In my division, there's a whole training period to become a department head, if you show interest and you are a good worker, they will encourage you to go to the training.