hi there, Tom
the default ringtone is set at "Settings; Sounds; Sounds and Vibration Patterns; Ringtone". The factory default ringtone is named Marimba.
So, if I understand you correctly, you have set an individual ringtone for one of your contacts and when this contact calls you, the ringtone is not sounding at all?
Is the tone you want to assign listed in the above mentioned settings page (at the top of the list)? If so, does it play when you select it from the list? If not, try playback in iTunes. If that tone fails to play in iTunes as well, it appears that the tone has not the required format, try to create it again. The tones bought at the iTunes store should work.
I assume that you have not switched the ringer off and that the ringer volume is on a hearable level.
regards, Chris