That's normal. Great Planes speed controls have an arming sequence. You need to plug in power with the throttle stick all the way back, then move the throttle stick to full for 2 seconds (if you can see the speed control, it will start to blink now) then move the throttle all the way back again. Once you do this, the throttle will operate normally.

This detail should be in the instructions that came with your slinger's speed control.