Hi,
There are two things that happen a lot. First is that the channel is not assigned to the switch you are using properly. Have you checked to see if a servo works on this channel the way you would expect with the switch? Second is that the travel is not set at 100% or greater. You might also try putting on a channel that you know is assigned, like ailerons or rudder, and seeing if it come on when the travel is set at 100% or more (usually high rates). If all this fails they you may have a failed transmitter but this is very uncommon. Please let me know if you have any more qustions.