Hello, I hope the following is of assistance.
The music on the USB stick needs to be in the correct format MP3. If I recall this correctly then music downloaded from i-tunes isn't in the correct format to play via the USB on this model Fiesta.
As the Bluetooth is OK and if your i-pod works OK via the Y lead through the AUX and USB ports then this would indicate a format concern.
Try an MP 3 file and go from there.
Let me know if I can assist further or if you have more questions and I'll reply as soon as I'm able to.