Final Fantasy 7: Have I lost Yuffie?
Solution 1:
You haven't lost her forever, but there are very specific places that she'll show up...if she isn't showing up in the one you've been encountering her in, maybe try one of the others?
Here's the wiki page with the locations (it also gives the correct answers):
http://finalfantasy.wikia.com/wiki/Yuffie#Acquiring_Yuffie