Going up in voltage is what I also would try next. I'm not sure about this plug, but I have used some in the past that were 2volt plugs. Being a thicker element, and depending if it is platinum or tungsten, would make a big difference in the voltage needed. Not all mfgs state their plug is 1.5v or 2v. It's just assumed its 1.5v.
What is the heat rance of the plug? Cold, Medium, Hot? Requirements for the different heat ranges have been known to differ.
If all else fails, contact Fox directly and see what they say.
http://www.foxmanufacturing.com