Here are some steps you should follow:
step 1:
Connect ReaSnow S1 to PC while holding the button. Release the button until the LED indicator is stable green.
step 2:
Then run "flash tool" on PC,and click "Upgrade firmware" button.
and you can also try some other USB cables or USB ports on your PC.
or it might also be caused by a defective mini usb port on ReaSnow S1.