iPhone6 - My purchased ringtone is selectable and plays when selected in settings but isn't used for the alert.

Why won't my selected alert tone play when a message arrives? It used to work and then stopped for no known reason. The tone will work on e-mails, etc. but when selected for text messages, the 'chime' sound plays instead of the purchased (off iTunes) sound (substitutes the chime sound). I can change it to other standard sounds, but not the one I purchased. I hoped that updating to iOS 9.0.2 would fix the issue, to no avail. Any ideas?