Hi HarryTheCat,
I don't know why you're having trouble but it works fine for me. I have a general ringtone for most calls (Settings & Tools > 4. Sound Settings > 2. Call Sounds > 1. Call Ringtone) and I have a unique ringtone for my wife (Contact List > [my wife] > Ringtone).
When my wife calls, I hear her unique ringtone. When someone else calls, I hear the general ringtone. It works great.
Best regards, WiredNot