assigned ringtones not working

I have a 3gs (AT&T). Assigning a specific ringtone (whether custom or default with the phone) does not work. I have created a custom ringtone and can choose is as the default for all calls and it works fine as do any of the ringtones listed under sounds. I just cannot successfully assign a specific ringtone to a specific contact. Have reset; checked for duplicates; scoured the internet and found nothing that works. The phone software is current. Any suggestions would be appreciated.