Just have solved the problem, but dont know why.
Open the echo_lo_L and echo_lo_R at LeapMotionContent/Character/Echo, popup the message:
The preview mesh hierarchy doesn’t match with Skeleton anymore. Would you like to regenerate skeleton?
Click Yes, and save the two, then open hands_lo_Skeleton, same message popup, click yes and save, if one file can’t save, use File->Save All or ctrl+s. Then, never touch these assets again, it will works fine.