Maybe you could try to whip up a test sketch for the Shield shield where you use Serial1 to talk to itself (TX -> RX) or to Serial2 (and back) and send debug outputs over Serial to your PC - without your other hardware.
This way you might find if the serial communication works at all.
You could also test your shield pins whether they have been frazzled somhow (digitalWrite() and digitalRead() work on RX/TX just the same).
You could also look here