I have this with two HTC One S phones now:
Latest HTC Drivers installed
When in Recovery, I can connect via ADB
Fastboot always says USB device not recognized
I have tested this on 5 different computers now
I have tested with various cables.
I am guessing that the Lolipop upgrade that I tried on these phones must have done something to kill the fastboot connectivity.
Unfortunately, the ROM is not in a workable state and because the phones are s-on I can't flash the boot image.
Therefore, the phones are pretty much bricked unless I can get a fastboot connection. Without that, I can't even run a RUU factory update because the software does not recognize the phone is attached.
If you've got a solution I haven't tried, I'd love to hear it.