You are probably trying to measure with a volt meter.
The signal sent to the servos is a pulse. For 'center' it is a 1.5ms pulse every 20ms. So its only high for 7.5% of the time. So if you are using 5v then 7.5% is about 0.375v (forgetting about mean vs RMS etc).
So it is expected that you would see a small voltage.
The difference between full-reverse and full-forward is only 0.5ms either side of this. So the voltage on meter will only vary by a very small amount.