“Friction” is key to the proper function of friction shifters! I’d check to make sure that the shift lever assembly is properly assembled and that the contact surfaces are clean, smooth and free of lubricant. As mentioned, some shift levers have a D-ring or tiny wing nut like arrangement to allow adjusting the degree of friction that holds the level in any given position. As also mentioned, if “finger tight” isn’t sufficient, something’s not right.