Pressure!

People are different in how they respond to pressure placed on them by others (e.g. coaches, bosses).  For some it is a necessary motivating factor, and it is the drive that without which they would under-perform.  However, for others it has the opposite effect.  

There are those who are self-motivated and who constantly seek to improve their skills.  These individuals often put pressure on themselves to achieve.  When pressure is added to this type of person, the result is an under-performance, with a higher level of frustration and ultimately disillusionment.  

There is a balance that can be reached.  Motivate the person who needs that motivation, but discuss what you need from the individual who places a lot of pressure on themselves to succeed.  

That individual will apply his/her own style of motivation and will perform to their highest level when left to their own devices.  The key to obtaining the most from players or employees is about understanding them, knowing their motivations and working with their innate characteristics.