power isue with I2C

Hello everybody

The situation is the following:

I have and Arduino Mega (Master) connected via I2C to a Arduino Uno (Slave). If the Arduino Mega is powered via USB, the slave works perfect. However if a power the Mega through the Jack (12 Volts) the slave don´t do nothing.

Can somebody give me a hint about this.

Thank you very much

Did you also join master/slave grounds?