pinch rollers still leaving black lines on substrates

[gabagoo]

I have no idea what the black stuff is or how it gets on the rollers, but it is driving me crazy in the way that sometimes it happens and other times it does not. I tried cleaning with alcohol, but no effect. Any ideas on what causes this and how to remove it? cleaning fluid? or will that damage them?

 

[signage]

Alchohol on rubber isn't a good idea, it dries it out.