Hi,
I have the same problem. But I found that pressing the switch "Disconnect autothrottle" located on the left or right side of the throttle levers in the cabin brings the throttle to work manually well again.
For Boeing, this autothrottle should be engaged automatically before the takeoff to control throttle automatically during different flight phases. And it is possible that the automatic startup procedures in FS did turn it on.
For more info see https://en.wikipedia.org/wiki/Autothrottle