Are you using Windows or Linux? I gave an answer to rule out likely errors with communication between PC and your device. Perhaps I chose my words badly. It is a known issue that Windows will find drivers to run adb, but not fastboot, so you need to keep checking for drivers until you can communicate both adb devices and fastboot devices. (Same applies to the other USB issues I mentioned, in some edge cases) You can go no further till you get to that point!
Edit. Thinking that Windows is more likely here is one approach: Connect the phone to PC with usb, and open Device Manager. You will know you are looking for a against any connected device which has a driver problem. If you have the run Windows Update, ensure the warning clears. Boot into Recovery mode,
Device Manager still open. Now boot into Fastboot mode.
Should the appear at any stage - Windows Update again.