Try white spirit that might help but if not i would go back to the company you got the windows from, there was a main plastic supplier years ago that had a very bad product that went discoloured in the sun.

Our company have been arranging treatments for customers that have had this prob and if it hasnt been resolved within 2 treatments then the windows have been replaced.

Best bet is to go back to your orriginal supplier and find out what there policy on this is.

Hope this helps