Why is my wig cap staining my dolls head?

Hello everyone! 
My poor obitsu girl Leelu is in bad shape.
Not only is her arm broken, her wig stained her forehead! I took her wig off and tried to wipe her head down, but it didn't work.
So I decided to just put her wig cap back on. But I took it off earlier today and her head is even more stained! I can't believe it! She has a silicone wig cap and it is still staining her head. I don't understand, I thought they were supposed to prevent staining from wigs! How could this have happened, does anyone know? And I plan on buying a pukifee, what can I do to prevent this from happening to her precious head? :<
When I get home I'll take some pics and post them up!

My poor obitsu girl Leelu is in bad shape.


When I get home I'll take some pics and post them up!