"If you want people to change, you must provide crystal-clear direction." ~ Chip and Dan Heath, Switch: how to change things when change is hard
"She's just lazy," is an easy assumption to make when an athlete doesn't meet your expectations.
"He's not working very hard," is just as easy an assumption when a coach has a losing season.
Both assumptions are possible, but unlikely. It's better to first assume that the athlete/coach/student/employee who "failed" didn't receive enough direction and support.
Conversely, it's worth speaking up if the assumed goal wasn't worth pursuing in the first place.