Arduino + SIM900

Hi,
My proyects works with Arduino Due and a SIM900 Gprs shield.
I've a problem. My proyect works fine when I put the arduino connected to my pc with a USB.
The problem comes when I connect the arduino to a power supply, the SIM900 doesn't works anything.
I've tryed http://playground.arduino.cc/Main/DisablingAutoResetOnSerialConnection
But it does't work neither.