Sometimes, I have run across hair that selecting a Display Mode to reveal the texture in preview mode will not work.  However, quarker's pose "Display Mode Restorer" usually works in those cases.

http://quarker.blog5.fc2.com/blog-entry-5.html#more  (Display Mode Restorer)

Just select the hair object and use this pose if you have problems getting the hair to show up even after using the Display settings.