I think this is related to chance cards, its a problem I am having and have searched and searched all the boards I belong to and have found no one else with this complaint. Many of my simmies work all the way to the top of their career and never get plat and still have the lifetime want to become what they already are.
I have been forced to try to get them demoted in order for them to get justice. After pondering why this might be occurring I remembered that at least one of them got to the top of their career via a chance card promotion. Could that be the cause of my glitch? I may have failed to send them back to work immediately in their personal vehicle and therefore the official promotional process never took place. Does that sound possible? Anyway the demotion-remotion strategy is a pain but it works.
Also, since I hate having my sims fired I always save before they go to work. If they lose all their dough, get demoted, lose skills I just suck it up. But if they get fired-I reload.