Check if your 2 speed clutch/changer is working properly.If it is,then you have to adjust it by screwing in the set-screw on it.In your situation,try screwing in the set-screw more,clockwise,it will make the clutch/changer shift later in the powerband enabling you to hear the shift.

Or check your 2nd gear spur,it has the little pin notched in the gear for engagement.Sometimes that pin wears out and the clutch/changer cannot engage/contact the pin.

Check the manual on page 8 for the clutch/changer adjustment.
Hope this helps a little.