Android : Cannot flash factory image to Google Glass

Sometime from last month, my Google Glass keeps rebooting "ok, glass" screen, and I cannot run and test my app.

I tried to flash factory image via adb based on the instruction below. https://developers.google.com/glass/tools-downloads/system

After I tried

$ adb reboot bootloader
$ fastboot devices
$ fastboot flash boot boot.img

the last operation (flash boot.img) doesn't return. Then I tried unlocking the device.

The first trial worked, and I got below response.

$ adb reboot bootloader
$ fastboot devices
$ fastboot oem unlock
...
(bootloader) Warning: Unlocking your device will void your warranty
(bootloader) and erase your personal data from the device.
(bootloader) Run "fastboot oem unlock" again to confirm.
(bootloader) Device still locked.
OKAY [  0.001s]
finished. total time: 0.001s

But second trial of unlocking failed like below (i.e. no response returned).

$ fastboot oem unlock
...

Is there any way to flash factory image on Glass under such situation ? Right now I have no way to make my Glass work normally again. Thanks.