how can I fix a dyson dco4 that wont pick up dirt?

My dyson, dco4 purple upright vacuum cleaner, the roller doesnt turn, the belt seems in good condition, there is no debri or string or other carpet debri caught in the roller, which moves freely when turned by hand. The upright sounds normal. When switched on everything looks normal, and when the cylinder is removed and filter under it, the suction seems fine, but there is no suction or movement of the roller, why? can you please help?