Are you taking about the default DFU bootloader?
Tell us more about your hardware -as Clive said, tell us how do you power the board, check physically if all power and ground pins (including VDDA) are connected. Tell us how USB is connected, check for bad solder joints. Tell us how do you manage BOOTx pins. How are other boot-related pins connected, can't the boulder go falsely to other protocols. What crystal are you using. If you can program the boards in other way, check if HSE and USB are working.
JW
