I work in a retail store. I have been pushing for promotion but it isn't happening. I'm told that I'm too valuable in my lesser role. 
I have proved in my worth when my manager is absent, this was acknowledged, but am still side lined as not having the 'right' skills. They won't train me for the more permanent higher paid role of supervisor and say I would need to develop the supervising skills to take on the role. 
I know there is someone being recruited as a supervisor who was an external candidate who therefore will need training! I will be expected to train the person though in all aspects of the store role just not the management side! 
So they won't train me but in one aspect but are intending to bring someone in who will need training in other aspects and of whom they don't know how effective a supervisor they are!
Now this person hasn't started yet and my manager is away for a couple of weeks.
Part of me thinks I should go all out for it and prove myself, my colleagues are highly supportive, the other part feels I would be a mug to do so!
What would you do?