Usually, this means that the host computer cannot provide enough power, or the port is too noisy. Sometimes this happens when you use a tablet, a laptop, or a desktop's "front panel" USB ports, as those often use a plain ribbon cable to connect to the motherboard, while they should use twisted pair cables with precisely matched lengths between data+ and data-. You should try to connect JeVois to ports that are directly attached to the motherboard (usually, in the back of the computer).
Please see http://jevois.org/doc/UserConnect.html for more on power.