The plug can have conducting deposits on it, and at .025" perhaps the current is leaking through the deposits instead of jumping the gap at high pressures. I once read a study that showed that most "cleaned" plugs required more voltage to fire than a dirty plug!! I would try a new plug. The RCEXL should work fine at .025". Load check your battery, and/or check it's voltage with the engine running.

If it just doesn't want to work properly with .025" after doing the above, hey, just set it at .019" and run it....

AV8TOR