You've tried downloading from the same computer to both boards? And one works and one doesn't?
When you changed the baud rate, you did by editing the serial.download_rate in the preferences.txt file, right?
On the board that doesn't work, does an LED on pin 13 blink when you reset the board? How about on the board that does work?
Try swapping the chips between the two boards. That should tell you if it's the chip or the board that's not working. If it's the chip, then it probably need to have a bootloader (or a newer version of the bootloader) burned on it. Or maybe's it just dead. If it's the board itself, you'll need some advice from the Arduino hardware experts (though flipping the board over and checking for bad or loose connections is usually a reasonable way to start).