Android Fastboot devices not returning device

For Windows:

• Open device manager
• Find Unknown "Android" device (likely listed under Other devices with an exclamation mark)
• Update driver
• Browse my computer for driver software
• Let me pick from a list of devices, select List All Devices
• Under "Android device" or "Google Inc", you will find "Android Bootloader Interface"
• Choose "Android Bootloader Interface"
• Click "yes" when it says that driver might not be compatible

Are you rebooting the device into the bootloader and entering fastboot USB on the bootloader menu?

Try

adb reboot bootloader

then look for on screen instructions to enter fastboot mode.

TLDR - In addition to the previous responses. There might be a problem with the version of the fastboot command. Try to download the newest one via Android SDK Manager instead of default one available in the OS repository.

There is one more thing you can do to fix this issue. I had the similar problem when trying to flash Nexus Player. All the adb commands we working fine while in normal boot mode. However, after switching to fastboot mode I wasn't able to execute fastboot commands. My device was not visible in the output of the fastboot devices command. I've set the right rules in /etc/udev/rules.d/11-android.rules file. The lsusb command showed that the device has been pluged in.

The soultion was quite simple. I've downloaded the the fastboot via Android Studio's SDK Manager instead of using the default one available in Ubuntu packages.

All you need is sdkmanager. Download the Android SDK Platform Tools and replace the default /usr/bin/fastboot with the new one.