Like puppyz said, most likely fouled plug. This happens if you fly around all the time without leaning the mixture. You get carbon deposits that prevent the electrode from firing properly. You can usually burn off the crap by running up the engine and leaning out the mixture a whole lot to increase the temperature enough so that it clears itself or if you have a lot of time to spend you can change plugs. If it is really really badly fouled you may not be able to burn it off in which case you are only left with the taking the plug out and changing or cleaning it manually.