I know that this is a resurrected old thread, but GoldenNibs is right -- it could have had paint or India ink or something in the pen.
Don't know if the OP is still around 6 years later, but my advice would be to use Rapid-o-Eze. Apparently that will work when nothing else does. I used to use it all the time to clean out my Rapidographs, back in the day.
Ruth Morrisson aka inkstainedruth