If your boards start acting weird, such as not responding to the code, we suggest to re-burn the bootloader from the Arduino IDE. To do so, you will need a 6pin (two rows) pogo pin, and a AVRISP-mk2 programmer.
1. For the main board, the contact area is on the front side of the board. For the other boards, flip them so you can see the contacting area on the back of each board.
2. Install the AVRISP-mk2 driver in your computer. Connect your AVRISP-mk2 programmer to your computer. Then plug the pogo pin into the female header of the programmer.
3. Now press the pogo pin exactly on the contact area of the board. You should be able to see your programmer start to blink orange a bit then turns into green. If it turns into red, try to reverse the orientation of your pogo pin.
4. Keep pressing the pogo pin. In your Arduino IDE, click Tools -> Burn Bootloader. In 2 -5 seconds, you should be bale to see a message like this. If you got errors, please make sure that the pogo pin is in good contact with the copper area. And make sure that your programmer works properly.