I have the same problem. Mobile POD app works perfectly but Garageband does not recognize Sonic Port / guitar input (the indicator at the top left of Guitar Amp with the symbol of the jack does not light up).
When I connect the Port to Sonic 'iPad the red light at the top of the Port Sonic turns on and then off. The same indicator of the jack on the interface of Guitar Amp on Garageband.
Please help - Garageband interface is also for me a key reason for buying Sonic Port !
Thx