many possibilities, is this while you are at a dead stop or rolling?
if its rolling a maladjusted chain can make it difficult to shift or hit neutral
low oil,
bad oil
bad clutch plates
bad shift fork
bad shift barrel
worn synchro's
poor hydraulics
worn clutch rod
loose clutch case (my busa had that new out the door from dealer, rode about 8 miles and the bolts backed out to the point I couldn't shift or had no clutch)
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