Fastboot doesn't recognize Nexus 5 on L I recently flashed the dev preview of android L on my Nexus 5, however, I did what is called a "dirty flash" - I just used TWRP and flashed the image as if it was a normal ROM. The result is that it's pretty bugged right now - many apps just crash or don't work correctly.

I'd like to do a proper install now, but the problem is that fastboot doesn't see my device, though adb does. Typing any fastboot commands just result in a "waiting for device" message. On 4.4.4 I didn't have any issues with fastboot. What might be the reason?
adb reboot bootloader


then 

fastboot flash &lt;whatever-it-is-you-want-to-flash&gt;


If you see  at any point when executing fastboot command, the PC is not configured to use fastboot properly. Most typically, this is because you are not running fastboot with elevated privileges. It may also be that you do not have the drivers correctly installed, which may happen under Windows 8 because of its insistence on signed drivers. - Source