Check the supply of the module and the CAN bus connection. If a non-supplied CAN cable is used, a 120 ohm terminator may be missing. Set the correct baud rate, that means if the flashed program has an other baud rate than 125 kbit/s of the bootloader, it must be scanned at the baud rate of the program. Also, check the boot loader ID, which by default is 1FFFFFFX as a 29 bit ID. If it still can not be found, check if the module reports with the Bootloader-ID when switching on.