Actually, it isn’t broken, it’s just not clamped in the frame correctly.
My fantastic cycle mechanic, Aaron (http://fusionpeak.com.au) discovered that the curled side of the hub was slipping in the frame. So, if you have a Schlumpf hub and find that when you change into high gear the gear feels like it isn’t engaging (although it switches OK and works fine in low gear) it could be that the hub is slipping in the frame.
When you are in low gear, there is no torque on the hub against the frame. When you are in high gear your pressure on the cranks is transferred to the wheel by turning against the frame. To support this torque, one side of the hub has a knurled surface, which grips against the frame so that the hub can turn 1.5 times the pedal revolutions. If the screws holding the knurled side into the frame are not tight enough, the torque will overcome the friction and the knurled side of the hub will simply twist inside the frame clamp.
The simple solution to this is to tighten up the screws on the knurled side of the hub. An even better solution (or prevention) is to apply some fibre grip gel such as http://www.finishlineusa.com/products/fiber_grip.htm on the knurled side. Schlumpf manual specifies the torque that should be applied to the screws clamping down the knurled and non-knurled sides. Using the fibre grip gel will help avoid the need to over tighten the screws, which will cause friction and bearing damage.
Also, if you are in Sydney and need a Schlumpf wheel build or uni maintenance, I would highly recommend Aaron at http://fusionpeak.com.au