We live in an era of team-building and process-improvement in the business world, in which owners recognize the negative impact of poor development on employees. You can stack your office with the best looking resumes, but let them rot in a poor office culture and you'll see many employees leave or stop producing. That's why more businesses have started investing money to address these invisible developmental issues. You won't see bad habits, bad communication, lack of motivation, or failure to reward on any chart, but you can see the end results in declining dollars and performance. Talent can veer off course and be compromised if we ignore the people behind the jobs. That applies to all of us, but especially to younger, newer employees.
Same thing in the hockey world. If a young player was progressing well relative to his peers but then stalled, it's clear that something slowed him down. Bad habits, bad communication, lack of motivation, or failure to reward -- take your pick. If that player looks worse than he did when he started, the coach or trainer is usually at fault for ignoring his unique needs. Trying to squeeze him into the same developmental template is poor development. He might've needed more time here, more attention there, more work in that area, some intervention to keep him from falling off course. Going that extra mile increases the odds of a better player; ignoring those unique factors increases the odds of a developmental stall. Sure, there are players with irredeemably bad attitudes who ignore all advice, or players who physically plateau and are unable to progress -- those are the built-in limits that go beyond what any team's development can do. But when a player has the right attitude and demonstrates the skills at a certain level, there's rarely a reason for him to regress beyond an absence of development.
This will never be an exact formula, obviously, but if we have to take an educated guess, a player who arrives at a level and can't keep up was probably scouted poorly. On the other hand, a player who was able to keep up at his level but then dropped back was probably poorly developed.