Did you use something to hold down the cork? Or did it just flatten out?
A little bit of both:
1) I rolled the rolls in the other direction,
2) If 1 didn't work, I rolled it up, soaked it in water and put some elastic bands over the whole roll, then I let it dry halfway, unraveled it, and put something flat with mass over top to flatten it and then let it dry. The last part is important, as cork will expand and shrink with moisture.
I was worried about directly rolling it in the opposite direction as any cork stored for long periods of time becomes brittle and a tad fragile. The water helped alot.
Hope that helped.