Pwm is changing but servos don not work?

Guys I met a problem here:
I can see the pwm signals of servos are changing as I move the stick in the servo output inspector.
But actually the servos are not working when I move the sticks.
And I also add an extra BEC to the M output to make sure the servos get enough voltage while the uav is armed.
So guys can you give me some ideas to solve it.
Thanks!