First, make sure you actually remembered to set the fuses on the one that's not working (ie, maybe you forget to set the fuses for 8MHz, so it's still running at 1MHz). I would be interested to hear of an atmega328p that, at room temperature and 3.3~5v operating voltage, had an internal oscillator that was off enough for serial not to work (I've never encountered an individual chip that was - though such a chip would be entirely within spec).