Agreeing with what the guys have said, I would only add that you can tell if the car is attempting to regen by looking at the economy consumption figure on the centre display when the car is at standstill and idling. Normally, you get an instantaneous readout in MPG, bit at idle/standstill this changes to gallons per hour and will normally read 0.1-0.2 GPH. When regenerating this increases to around 0.7 to 0.8 GPH. If after a long run say in excess of 30 mins your DPF keeps regenerating, after several runs I would definately be suspicious of a problem. You need to keep checking this at the start, during and at the end of the runs.
If, as maybe unlikely, your car does keep trying to regenerate then their could be a blockage in either the DPF, the sensor/pipe or a pipe split.
If your car has been trying to regenerate over a long period then your need to check / replace your engine oil because when the car regenerates a small amount of diesel is absorbed into the engine oil. Often the engine oil level will increase, just something else to check. Richy