I'm wondering if you're getting enough heat on it. If you have a sock on an iron, I've found sometimes the heat coming from that combination isn't as intense or direct, as much as it seems without the sock on the iron. If you want to take little chance, try the iron turned up a bit more, without the sock in a very inconspicuous area, just to see if it might help.

Whatever you do, stay away from the areas where colors meet. The colors/film will just shrink away from eachother, and possibly open up a seam... or skew any straight line(s).