FLX6 Disconnecting(When open fx)

I am Flx6 user for 1 year, in this month have a big problem, when i use fx, flx6 will disconnecting immediately, but this problem not often, sometimes open fx 10 times will disconnecting, some open fx 1 times will disconnecting. I try different ways to solve but failure to complete , eg. trying a different USB port or USB cable. Hope somebody to help me.