Hello,
We have been working on Nordic nRF52-DK board.
Board can be reachable by Bluetooth in the first 10 minutes of power supply. However after 10 minutes, we can not reach it by bluetooth and even we can not see the device by Bluetooth Low Energy Scan commands of linux. In this moment, we are sure that the device is working since it's lights are on but can not reach it.
Have a nice day