Make sure....
1. the height is properly adjusted
2. you have the brushroll nozzle locked in place and the belt on the shaft properly
3. it is NOT on delicate mode
4. the belts are genuine OEM and not generic (OEM belts with Kirby are not marketing, they do have a special rubber type that can stand up to the high RPM - generic belts cant)
5. the belts are not old stock
Other than that I cannot think of much else other than the fiber type of your low pile carpet is catching the brushroll bristles and is stalling out the brushroll.
I have low pile in my basement and my G4 and Ultimate G work flawlessly on it.