DIAGNOSIS: The motorized floor nozzle has a safety clutch that disengages the brushroll if it encounters too much resistance. On thicker carpets, hair and thread can bind it up, or the carpet itself might be just dense enough to trigger it.
FIX: First, flip the cleaner head over and cut away any hair, string, or carpet fibers wound around the ends of the brushroll. Pull it all out. Then, check the setting on the cleaner head itself. There's a dial—make sure it's set to the 'carpet' setting, not 'hard floor'. The carpet setting lifts the nozzle slightly for better clearance.
IF THAT DIDN'T WORK: The issue might be the battery. When the battery charge gets low, it can't supply enough power to keep the high-torque brushroll motor turning under load. Put a fully charged battery in and try it again on the same carpet.
HEADS UP: Regularly cleaning the brushroll isn't just for performance; a bound-up brushroll makes the motor work harder, which drains your battery much faster on a single charge.