The issue that creates this problem is worn shift levers . The nut holding the worn thin shifter lever on is bottomed on the trans fork lever before it completely compresses the shift lever. Buy new levers and nuts. This may solve your issue . If not you have worn trans fork levers. Anything else may work for a little while but they are just a temporary fix.
The shift fork levers must be tight before the nut is installed. Also the trans fork lever flats should protrude out of the shift fork lever. The nuts should have a built on serrated washer.