Please tell us what OS are you using, fastboot version.
Have you tried "fastboot devices" prior to launch the final unlock command? Maybe your PC to Phone connection was not ready, or you were in the Bootloader (you have to choose FASTBOOT from the Phone bootloader).
Another good thing to do would be to make sure that, within your Phone's Android, you have disabled the Fast Boot option. Phone-> Settings -> Power -> Fast Boot -> Unchecked