first time rooting a phone... (sony Xperia Z5 Compact)
anyways.. i had the same problem.. was stuck for 30 minutes!!
The way i solved the issue was to unlock the bootloader. after that it worked perfectly.
no need to change cable or try different port.
You can unlock bootloader developer . sonymobile . com / unlockbootloader / unlock-yourboot-loader (am not allowed to post links.. sorry). at the bottom, select your device and click continue.
then just register and check your mail. then'll you'll be given instructions on how to unlock bootloader.
hope this works for you guys, because it did for me