hi
the dpf block up because the dpf does not get hot enough to burn off the soot inside
if you drive and keep the revs very low, you will always have issues
speed is irrelevant, its revs it needs, so when you on the motorway, keep it in a lower gear so the revs are higher than normal, this will heat up the dpf and allow it to burn off the soot
if its been revved and still having issues, there maybe other problems like split hoses that are stopping it from regening, but the garage should find this
cheers
Joe