Hello, I hope the following is of assistance.
If the DPF fails to regenerate it blocks up very quickly, as you do motorway driving regeneration should be no problem therefore the reason for the excess soot load so quickly needs to be investigated.
First is to make sure the glow plug/fuel vapourizer is working correctly, if this fails the exhaust won't get hot enough to regenerate the DPF. If this is OK then the air intake system and injectors need checking along with the exhaust gas temperature sensors and DPF pressure sensor and hoses.
If the injectors are allowing too much fuel in to the engine this will block the DPF as it can't cope with the excess soot, on an older diesel engine this would be noticed straight away as you would get black smoke but as there is a DPF fitted this does as it should and collects the soot so you see no black smoke therefore you have no indication there is a fault other than the DPF blocking.
The DPF blocking isn't a DPF fault it is a consequence of a fault elsewhere in the system.
Start with these points, let me know if you have further questions regarding this and I'll reply as soon as I'm able to.