What is your speed when you press Set or Resume? The Volt may have a minimum speed below which cruise control cannot be engaged or re-engaged.
I have adaptive cruise control (ACC) and it can be balky sometimes. First time using ACC on a given trip, to set the ACC the Volt must be traveling at least 25mph. If I brake and come to a complete stop usually I can press Resume and the Volt will re-engage the ACC from either a full stop or any speed even if the speed is lower than 25mph. Sometimes it takes a second attempt to Resume ACC while I am manually accelerating. I am not sure if this hesitation is time dependent or speed dependent. Sometimes I have had to turn off the ACC and turn it back on to reset. Usually the ACC works fine.