one motor cannot move

[zsktju]

After assembeling the follower and the leader arm, i finish the calibration. But the second axis of the follower arm cannot move. The motors are all powered, since the light of the motors are on. And i check the position of the correspoing motor in the second axis by FD. The positon of the motor position in the second axis of the follower is 1976. And the positon of the motor position in the second axis of the leader is 1864.
why this happen? How can i fix it? thank you.