Ensure there is nobody connected to the CORE-XX. This can be checked selecting the Bluetooth input in the CORE device. If the blue LED is not blinking, there is some device connected to the Bluetooth Core device. It is necessary to disconnect this device in order to establish a new connection. It is possible that a previous Bluetooth pairing has expired. In order to redo this pairing, it is necessary to unpair and delete the CORE-XX from the settings of your device. Then, a scan is required and finally pair again with the CORE-XX with the corresponding PIN number.