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Joystick CH 6 CH 8

I’m working with an apm and arducopter 3.3 and I’m trying to configure the joystick.
In particular, I use a T. flight stick x.
I’m having trouble configuring channels 6 and 8.
The X and Y axes work properly when they are assigned to roll, pitch, channel 5 and 7, but when assigned to channels 6 or 8 they cover a very small range (a few points).
How can I solve?

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