Fighting spirt comes within the player.  Most of the time lack of fight can be a lack of confidence, work on their skills and conditioning.  Also, be sure not to place them out of their skill level in match play, you may need to drop them down a level to build confidence. I had a player who was shy and didn't seem to care about losing and didn't seem to try, she now is a high school player who has a fighting spirit and just loves to win.  Did I bring that about or did she just grow into it, maybe a little of both. Bottom line good coaching skills and time should work.  Good Luck.