the root cause of problem is having a unprofessional and incompetant manager.
When to distribute the work, he selects the best he have to give the toughest works. but when it comes to submit year reports to management regarding performance appraisal, he rate each and every member of his team equal.
The basic reason is the insecurity within themself, who dont want to allow the management to notice the talents and let him progress high.
poor chap dont understand the basic theory, if your subordinate gets promoted, it automatically push you too upwards.