You need to get to the bottom of this situation before you completely hit bottom. If you are in fact performing well on these higher level responsibilities, it makes no sense for your manager to be removing them from you. One question that immediately comes to mind is whether your manager thinks you are handling them well. If it is possible that he does not, then you need to launch a publicity campaign that touts your accomplishments.
When management starts peeling away responsibilities and offers neither explanation nor enhanced additional responsibilities, there are typically two primary objectives. The first is to get the work done correctly, and the second is to set the stage for you to leave, either on your own or ultimately through termination.
As your job morphs into routine drudgery, management may be hoping you will walk out the door. This is a tactic that is not free of potential exposure for management, but some still try this strategy initially.
In addition to your internal publicity campaign, you should sit down with your manager and discuss this matter. The fact is that your job is winding down, and you need to find out if your tenure with the company is winding up.
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