Disconnect the shift linkage from the shift lever. Move the lever up and down.If it will not fall after raising the lever by hand. If it is not free, remove the pivot bolt and lubricate the bushin the bolt goes through.
If it( shift lever) moves freely, suspect something inside the shifter housing.
p.s. the shift lever bolt nut is a jam nut. The frame is threaded where the shift lever bolt goes through.
My bike sat for 4 weeks waiting on a shift spring to find the shift lever pivot was sticking.