You have maxed out the gradient by following what cspear said to do. The flame was already self-lit quite well. The problem was the rest of your objects and environment are much too bright, combined with the fact that Poser "atmosphere" does not "glow" from self-lit props.

Your magenta flames are now mostly uniform maximum magenta and that isn't going to look good. You want contrast and gradient, not absolute brightness. Darkness reveals light - it doesn't make light darker, it makes light lighter. You need more darkness in your image, not more lightness. You should not see more than 1% of white pixels.

---

Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)