if the dpf's are blocked, there not going to clean easy, they need to get really hot so the burn off the build up
had a friend recently his played up and the dpf pressure differential switch pipe split, which caused the car to stop doing regens and long term blocked the dpfs
he had to take them off and had them cleaned by a company thats specialises in dpf cleaning
it cured his and was far cheaper than replacing
dpf aditives usually work by making the engine run hotter, mainly the exhaust gases, so the dpf get hotter and burns the soot out, ideally you need to give the car a good run with the aditive and its not speed you need, but more revs, keeping the car at higher revs, will get the dpf hotter and help to clear it
cheers