"Personal" Ringtone keeps dropping

Hello,

My new (brand new!) iPhone 4S seems to dislike my "personal" ringtone!?

For years the old 3G iPhone chugged along with the same personal ringtone without a problem.

During the week I've had my new 4S, the ringtone has reverted to the default ringtone at least 3 times after which I go into iTunes/tones and re-tick my personal ringtone, Apply, re-sync and all is well until next time.

Any ideas what I'm doing wrong?

Thanks

Garoolgan