Pasta Machine Question

I have an Atlas and even though I clean the rollers with alcohol daily
and keep the machine covered with Saran wrap when not in use, I find
that sometimes when I put clay through it that I get these little
black smudges of dirt in the clay. Sometimes they are small streaks,
sometimes they look like little bits of string. What puzzels me is
they appear randomly and just every now and then - you'd think that
once I picked up the dirt by running some clay through it once or
twice, that would be it but no matter how thoroughly I clean, it still
appears.

Has anyone else experienced this and did you come up with a solution.

Thanks
Rachel