This sounds like a simple fouling issue. Is the plug black? Wet? Are you putting the proper heat range plug in it? Make sure you read the manual or buy it at a motorcycle shop so you get the right thing. Just because it fits, doesn't mean it is the correct heat range. Too cold of a plug will let deposits form on the plug leading to fouling. If the plug is proper, you may be getting too much fuel in there and not enough air. That's either because there is too much fuel, or hopefully not, oil getting into the combustion chamber. If you see white puffy clouds when you start her up, go find the oil issue. Otherwise, check into the air/fuel delivery system and see what's up. First: If your air cleaner is too dirty, or overoiled you will restrict the air available and richen up the air fuel mixture. Afterthat, is the carb flooding over or anything? If the fuel is running nor stop through the carb to the inlet tract that could do it.
Last tip: Don't bother replacing the plug unless the electrode edges are rounded. If they are nice and square still, just take a folded over piece of sand where the electrode meets the porcelin insulator. Removing the deposits right there will take away the conductive film and allow the plug to work as it should - until it fouls again of course. Spray it off with carb cleaner to get that sand and stuff gone. That could save you a good $20 in plugs until you get this thing figured out. Enjoy! Jon