I have a DC35 Animal too, with the same problem. The little motor in the brush-head is very sensitive to low voltage and will stop if it doesn’t get enough “juice”. There are 2 reasons (in my case) that seem to affect the amount of power delivered to the head (assuming the battery is charged). 1) A filter that is slightly clogged will cause the main motor to draw more current, thus depriving the brush motor of adequate power. Clean the filter with running water and let it dry. 2) Dirty electrical contacts. When you separate the motor and battery assembly from the rest of the cleaner, look for 2 little gold-plated contacts. If they are even slightly dirty they will act as a resistor, limiting the current that would otherwise be available to the brush motor. Clean them with a pencil eraser.