Reflecting on her astronomic career rise, one of the leaders I coached once noted, I have always done better when my direct line manager was basically absentalternatively, they were mostly a barrier to my career advancement.She is not alone.At most companies, there are at least some managers who depend on their direct reports so muchnot least because they are actually doing their workthat they end up holding them hostage from moving up.There are also plenty of examples where the same talented employees transition from enjoying star treatment and executive sponsorship to becoming the very target of their line managers, who feel threatened by and jealous of their success, which they are eager to block.This goes beyond the anecdotal, as many scientific studies provide consistent evidence to explain why bosses are often the exact opposite of a champion, mentor, or sponsor to their direct reports, even when their sabotaging and boycotting goes undetected.Reason 1: Your boss doesnt want to let go of a high-performing employeeAs I found in my book The Talent Delusion, any manager who measures team productivity or collective output reliably will find that a vital few individuals in their team account for a disproportionately high chunk of the output. Just like its essential that they keep such individuals engaged, which may include giving them the star treatment, it is important that they retain them. But, ironically, being part of the vital few also makes you a high potential for the next career level and promotion, which will probably handicap their existing teams, and reduce the accomplishments of their existing manager.Unsurprisingly, research shows that one of the main reasons bosses become blockers of their reports career progression is their unwillingness to sacrifice or compromise their own team (and in turn individual) performance for the good of the employee or the organization.Reason 2: Your boss is blocking your career advancement as a part of office politicsAt times, career blocking may just be the product of wider organizational politics. For instance, even if your boss doesnt mind losing you despite the fact that they see you as one of the key members of their team, you may be hit by friendly fire if your boss is in a turf war with your potential new manager: think of it as your current and potential future boss fighting over an asset (you), not because they necessarily care about that asset, but because they are locking horns in a battle for power, influence, resources, and status. Its a bit like if you are leaving your husband or wife for someone elseusually a painful event for thembut that someone happens to be their nemesis or archenemyan unbearable provocation.Reason 3: Your boss is waiting for retirement Other times, managerial blocking may be due neither to fears of losing a star performer or vicious organizational politics, but simply due to existing retirement cycles, coupled with an unwillingness to distinguish between tenure and performance, not to mention potential. In other words, most people get stuck because their boss is waiting to retire, even if they have more or less retired from their current duties and role, albeit informally. As Max Planck, describing this in the context of academia, noted that science progressed one funeral at the time.To be sure, many tenured and senior leaders (and employees) are among the top performers in an organization, so there are many arguments to keep them for a long time, even before they reach minimum retirement age. That said, they may still be blocking or delaying up-and-coming employees from gaining a well-deserved career progression, which may risk losing them to other organizations, including their competitors.Reason 4: Your boss isnt willing to fight for you Managers may not be deliberately blocking their employees career advancement, and yet passively contributing to their stagnation. As we know, identifying high-potential employees, which includes the selection of potential future leaders and executives in the most successful organizations in the world isnt a science, but a mix of science and intuition.The intuition part includes the politics of championing and sponsoring people, especially when they report to you. It may well be that your boss likes you, values you, and has no objection to your advancement; however, they may decide its not really necessary for them to fight the heated battle for having ones own team members promoted ahead of those of your peers.While other bosses may actively campaign for their employees to be promoted, your boss may think that your achievements should speak for themselves, and that in a normal and rational culture, leaders should be able to make evidence-based decisions on career progression, rather than base it on popularity vote or who has the loudest and most powerful champion or sponsor. Sadly, your boss may be rightlogically and ethicallybut you will nonetheless lose out to some peers who are endorsed by politically active and powerful bosses.Reason 5: Your boss is a narcissistA final reason may be sheer narcissism, particularly vulnerable or insecure narcissism, which is not uncommon among bosses. Interestingly, narcissism may propel bosses to hire people who are just like themselves, and also designate them as successors: look at this brilliantly talented employee I brought into my team, they are amazingoh and they look much like myself.However, when those very bosses feel a competitive threat from those employees, or that more attention is on them than on themselves, they may get defensive and decide to retaliate. Imagine, for example, a manager and a right-hand employee who in some ways resembleat least from a personality perspectiveDonald Trump and Elon Musk; regardless of what you make of their talents, you can see how such romances may be short-lived, and how the amazing highs may be followed by incredible lows.Perhaps it is useful to remember that leadershipthe art of influencing others so they can collaborate effectively and form a high-performing teamis not just about impacting the people who formally report to you, but also your peers and bosses.So, just like we would expect a good boss to avoid the traps discussed above, we would also expect talented and high potential employees to be astute and politically skilled enough to persuade their own bosses to let them go, especially when that is likely to contribute not just to their own personal career success, but also the success of the organization. Managing up, then, ought to include persuading your boss to not be a barrier.