all the above would probably be your last resort possibilities to avoid having to take everything apart.
check the simple things first.
it sounds to me that it could just be your high concentration of oil to fuel mix causing one of your plugs to clog up, why not both i'm not sure. could be one of other guys' ideas above as to why...
another option could be that as a result of trauling, ie. low revs on the motor, then if it's a 2 stroke that could be your cause as two strokes run best when reving fairly high. so if you were trauling for a while then a bit of oil could have not burnt and staying on the plug.
30:1 is fairly rich seeing as the recommended ratio is 50:1 so i'd say give the plug a clen in white spirits, chech the obvious things, whack the plugs back in and try a leaner fuel mix. unless of course you think continuing to run it is not a healthy option for your engine.
goodluck champ.