There could be a bad solder connection on one of the Mega boards.
Try connecting a resistor/LED combination to SDA and SCL, and check their function as standard digital outputs (use something like the Blink example).
There could be a bad solder connection on one of the Mega boards.
Try connecting a resistor/LED combination to SDA and SCL, and check their function as standard digital outputs (use something like the Blink example).