Coaches generally judge on how a person trains and not how they play.
A parent may feel their child is a talented player but is the coach seeing that at training? Or is the coach frustrated by the child nattering away to her friends and ignoring instructions and generally slowing progress through the session?

Players with good skills aren’t always good team players.
A child may be the star of their school team with great personal skills but do they play well as part of a team or do they try and win on their own? Hockey is a team game and a player has to be part of the team.

During matches some players pay more attention to the coach than others.
Unless it's an important match we generally try to give everyone a fair amount of pitch time. However sometimes a player won't follow the coach's instructions or insists they play in a particular position when the coach wants to play them elsewhere. This leads to the player being substituted and a more flexible player going on.

So if your child is frustrated or not progressing as well as they might ask the coach what the child needs to do to make better progress. Find out if your child is showing the commitment and flexibilty to achieve their potential.