thats cause wd-40 isnt really a solvent, and if it is.. its reallly weak. WD-40 is designed to displace water from something, thus preventing rust.
WD = Water Displacement
40 = They got it on their 40th attempt at making a solution
hence - WD-40
Just use acetone (whats in nail polish remover) very lightly, and make sure and clean it all off after. Something like Acetone or even just Paint thinner (100% Mineral Spirits kind) Wont melt plastic unless you leave it for over a minute, so if you just apply, rub, and dry/clean it after, your fine. its putting it on then leaving it there which will allow it to eat away at the plastic.