My players keep complaining about the rarity of the smg/semi & rifle bodies.

But i put them into the loottable, but they almost not appearing in boxes. Any solution to fix this?

My players are also complaining of this,  same items. 

You need to add them to the rarity list, but keep in mind that vanilla military crate for example can spawn with 1 guarantee smg body/rifle body/pipes/hq metal... then bonus 1 item from the loottable like ex. gun or electrical part or clothing or whatever + scrap.

In better loot if you make rarity to smg/rifle bodies to 1 still sometimes they won't spawn because if you set 3 items per military crate will pick 3 from the loot table for 1st sloth and for 2nd sloth can pickup clothes or tool+ electrical part and filling 1st sloth with smg/rifle body or pipes/hqm is not guarante.