The other day I had a problem sofa... 100% light beige acrylic... The rest of the sofa cleaned up well apart from the seat cushions and front areas where the back of your legs rest.
The areas looks darker, a sort of grey colour... Anyway I used a selection of cleaning products including solvents and it just would not budge.
I then tried MPower and left it for 10 mins, extracted and that did the trick.
Sometimes the most gentlest cleaner in the van works best depending on the situation