why do my ringtones keep resetting to default?

Sorry if someone's already asked this, but I couldn't find anything. On my iPhone 5, seemingly at rondom all of my custom ringtones and alert tones reset to the default. Yes, I'm one of those people who has a different ringtone for each family member and every so often they reset back to my phone's default tones. Has anyone else had this happen? Is there anything that can be done?