How many miles on bike? When was last oil change and what type of oil if changed? Also where is the clutch lever free play set (should be 1/4"-3/8" measured at the end of the lever). If oil isn't too old and clutch free play is fine maybe you're preloading the shifter without realizing it. If you have pressure on it already before you clutch and then try to shift it won't want to engage. Can fix that by backing of shifter (possibly need to readjust if your boots are too big) or by blipping the throttle.
