Tried to answer before, but I forgot something so trying again :-)

Each truck model has it's own def files for paint jobs and they need to be in the right folder.

So check if the one that isn't working is in the right folder, and check this line in the def file (ignore the "sunbelt" bit) if it's for the right truck:
accessory_paint_job_data : sunbelt.scania.s_2016.paint_job (is for Scania S 2016)
accessory_paint_job_data : sunbelt.scania.r_2016.paint_job (is for Scania R 2016)

def files for the Scania S go here: def\vehicle\truck\scania.s_2016\paint_job
def files for the Scania R go here: def\vehicle\truck\scania.r_2016\paint_job

There could be other issues, but that's where I'd look first.

well thanks for trying to assist. for the Scania S 2016 it does seem to be correct
"/vehicle/truck/upgrade/paintjob/scania.s_2016/izzyngs/accessories/acc_0.tobj"
""/vehicle/truck/upgrade/paintjob/scania.r_2016/izzyngr/accessories/acc_7.tobj""

atm your reply is hanging in the Steam system, but it seems you are using modstudio, though I'm somewhat familiar with that, without seeing the mod I can't help you any further.

edit: it's not the location where the paint job is that matters, I mean, it does, but more important is where the def files are and where they are linking to.

Well. I'll get it put up on a OneDrive then link it in here so if you want to have a nosey around to see where I've gone wrong then feel free