Many modern diesel cars are fitted with Diesel Particle Filters (DPF). These filters are designed to remove soot form the exhaust gases. These filters fill up with soot as they collect and they are designed to self clean by using a programme initiated by engine ecu to burn off the soot while you are driving. With many modern diesel cars, they are used for short trips and the engine never gets up to temperature so the cycle doesn’t start. Other issues can stop the cycle such as bad glow plugs, faulty glow plug modules, engine check lights etc. If the car cannot regenerate the filter the car will suffer lack of power and go into limp home mode.
Particle filters are expensive to replace and in most cars it is easier to remove the filter and remap the engine to not require this anymore.