^^ as Bonsy said, worn (grooved) clutch basket or slightly warped clutch plates can cause it to drag. It isn't uncommon for there to be a slight drag when the engine is cold either. I've found on few occasions that the inner most pressure plate (steel plate) in the clutch pack tends to warp first because it absorbs the most heat. You can lay the bike on it's side, pull the clutch cover, remove the clutch spring bolts and springs and the clutch pack will slide right out. Inner and outer basket will show some wear marks on the splines on the inner hub and where the "tabs" contact on the outer basket. If they are not too bad you can file them smooth. Inspect the steel plates for blueing and warping. Assuming all steel plate thicknesses are the same, take that inner most plate and swap it with one of the plates in the middle of the clutch pack.
Stock clutch baskets and hubs are usually a natural aluminum finish, if you see anything that looks billet or is hard anodized black or a dark color you most likely have some aftermarket components in there, which hold up a lot better than the stock unit.
