It seems that for some reason Futaba has restricted the maximum angle of deflection on channel 3 of the Skysport 4 transmitter. It is due to the small piece of plastic shown in the photo above. All the other channels have a normal deflection angle. Why they would do that is beyond me. Channel 3 is the throttle channel on an aircraft. Maybe something about airplane throttles I don't know?
If you are running an Original or Tiny Smartcontrol on this channel you will not be able to activate the accessory switch(s). On the Advanced Smartcontrol there is a special setting just for this channel, but on the older controls it was impossible to fix the problem due to lack of resources in the microprocessor. You have two options, either move to a different channel or remove the piece from your transmitter.
On my landing craft I tried having both the rudder and throttle on the same stick. Works great, I can hold the transmitter in one hand and control the boat with my thumb, while holding a coffee or beer or whatever in the other hand.
If you decide you want to take the offending part out, just take off the back cover, remove the small leaf spring which makes the clicking sound. The part you want is just under the leaf. Remove the 2 small screws and pull it out, then replace the leaf spring and the back cover.