Not sure but you are not hooked up to the usb at the time of the text are you? Personally I wipe out all the Stock Ringtones and Notifications and replace them with the ones I have collected. I find that by doing so and Keeping them all in the Systems media/Notification and media/Ringtones allows me to hear them when ever my phone is activated.
Also Try doing Wipe Cache and Dalvic Cache