Can't unlock bootloader, fastboot not detecting device

I've been trying to unlock my bootloader but have been having issues. I have done the following:

- Enabled Developer Options
- Enabled USB Debugging
- Enabled the OEM Unlock in the Developer Options

I have adb and fastboot installed from the android SDK. Using that, I have been able to see my device with 'adb devices'. I have also been able to reboot it using 'adb reboot bootloader'. However, once the phone is in fastboot, it does not seem to be detected by my machine. I have tried 'fastboot devices' to no avail. Using 'fastboot oem unlock' just stalls at '< waiting for device >'. Am I missing something?

I have also tried:
- Using a different computer
- Using different USB ports
- Using different ADB / fastboot package (the minimal one).

Please assist! I'd really like to unlock my bootloader now for future flexibility.