Most likely you are running lean - fatten up your mixtures a little - do a pinch test and retune the high speed.
One very cool, wet night, a mate and I fed several plugs though our cars . . . figured out later it was the cool air making the cars run lean . . .
We are also using 25% nitro, so went from a hot plug to a medium plug in cool weather - problem is sorted - we are no longer consuming plugs, they last for ages now.