Sounds like you have done the right things. Its important to clean the plastic well first. Its important to get the water hot enough to dissolve the dye if you are using the powder. I usually take it to a boil. I've heard that adding salt to the dye may help with some plastics. Heat seems to be the key for stubborn canopies. I reduce the heat until I can put in cut off test pieces of plastic without them curling. If you can keep the dye at this warm temperature it seems to work faster. If this fails you may have a plastic that is "dye proof". You can always fall back to the transparent paints the car guys use to tint the canopy. Spray multiple very light coats on the inside until you get the look you want.