You don't have a voltage problem--The receiver passes on whatever voltage you have from your battery to the servos---The maximum amount of voltage you will have to the servos will be close to equal to the voltage from the RX battery pack. (A slight drop is normal.)

You either have a pulse width problem from the RX or a severe binding problem...Take the servo loose from the valve---If the servo fails then you have an RX problem...If not then you have a binding problem...

Kevin