Because the DPF is under the car, not 'close coupled' in the exhaust manifold, the car needs to be driven quite a long way (50 miles) at 2000 - 2500 rpm to get the DPF hot enough to promote a passive regeneration. An active regeneration will happen automatically, but if the DPF is too stuffed up with soot for this to work, the post-injected diesel intended to fire off an active regen will instead sink into the sump.