As with any filter they have to be regularly cleaned to maintain performance. This cleaning process is called 'regeneration' in which the collected soot is burnt off at high temperatures.
Passive regeneration occurs automatically on fast A-road runs when the exhaust temperature is high. However, many cars don't get this sort of use and hence vehicle manufacturers design an 'active' regeneration processor.
When the soot loading in the filter approaches about 45%, the vehicle's ECU will initiate post combustion fuel injection to increase the exhaust temperature to optimum levels and trigger active regeneration.
A regeneration cycle takes place when driving a journey for 10 minutes or so at speeds greater than 40mph.