This article outlines the reasons an Android device might not be recognized in fastboot mode and how to fix the issue

If fastboot is unable to detect your Elo Android device or you are receiving the "Waiting for any device" message when trying to communicate the device, the required Google USB drivers are likely not installed. 

For Windows devices, you can download all required Google drivers via Windows Optional Updates.
1.  From the Windows Update Advanced Options area, expand the "Additional Options" section


2. Select all updates relating to Google, LeMobile, Android ADB, Android Bootloader, and Qualcomm.


3. From the command prompt, try to connect to the Elo Android device using the fastboot devices command.  If successful, the system will return the device serial number.