belt slippage?

I have a 06 kfx700. My question is that it seems like when I go to take off at half throttle I have to give it a considerable amount of gas to get it up to speed. Maybe I am starting to get used to how fast the bike is or it seems to me like the bike doesn't have as much torque. The owners manual says that the belt needs to be tightened after 100hrs an mine isn't close to that. I don't smell the belt burning or any white smoke coming from it when I am riding. When I removed the belt cover from the side of the bike there seemed to be little wear on the belt, but the belt seemed to have some play in it between the two pulleys. I don't know how much play there is supposed to be if any between the pulleys? If there is not supposed to be any play how do you tighten the belt? If anyone has any suggestions I would appreciate it.