I had the same probelm a couple weeks ago and I found, with my V8S, that if i had another usb stick plugged in the back of the box and tried installing the update from a usb stick in the front port, the two would counteract each other and the update wouldn't install.
So installing it with just one USB plugged in sorted that problem.
Also, deleting the existing channel list on the box before installing the new one also helps!!