I'm not sure if I've missed something. I've looked through all the tutorials on how to get the infinite roll to work and it's just not happening. Could someone offer some help?

Sorry to read on your post, please make sure the gimbal has been balanced properly and settings are correct:
1. In the Ronin app, go to “Configuration”---”Control Settings”--- “Channels”, set CH3 to “Roll” and set the rest channels to N/A.
2. Use the Ronin-S with Flashlight mode.
3. Push the joystick left or right (which is the CH3) to enable the axis to make 360 turn.
If the issue persists, you can try to raise the device by a unit (please refer to the following picture) or tilt the device 45°, then check if the Power value of the Pan motors is in the range of +/-5. If not, please balance again.
Please let us know if it helps. Thanks.

I was having the same issue until I realized the pan axis had not been properly balanced. See this video right around 2:00:

You absolutely have to have your gimbal perfectly balanced or it will not work