To resolve the issue:
- Make sure that "Sync ringtones" is selected in the Ringtones tab for your iOS device in iTunes.
- Sync your device and allow the sync to complete.
- If still unresolved, deselect "Sync ringtones" and click Apply.
- After the sync finishes, select "Sync ringtones" again and click Apply.
If your country offers music in the iTunes Store, you have the option to purchase tones for your iOS device(s) that use iOS 5.
* Ringtones can only be purchased on iPhone; if you want to know whether you can buy ringtones, see iTunes Store: Which types of items can I buy in my country?.