You can diagnose boot if you have a USB to serial cable connected to the 4-pin serial port of JeVois. First you edit boot options on the SD card to enable serial console. Then you should see boot messages. See here:
(edit uEnv.txt in the BOOT partition and uncomment the second set of kernel parameters, then launch some terminal program on your host, selecting the port of your USB-to-Serial cable and 115200 bauds).
But usually the problem is either a bad USB cable, weak USB port on the host, or the SD card is not fully inserted into JeVois (try to use a credit card to push it in all the way, just beware that if you slip, it will fly out, so be ready to catch it).