While gunpowder has been rightly characterized as one of the greatest leaps forward in the lethality of weapons, guns required some companion inventions to become the dominant small arms weapon system of the current age. Since I'd rather not have guns be the dominant weapon system in the world I'm building, I'm looking to block at least some of these ancillary inventions so that guns remain messy, relatively unreliable, and decision-demanding, leaving room open still for closing to melee combat as a practical matter of course.
In particular, one of the companion inventions I consider key is the percussion cap: a small metal piece that holds a small quantity of primary explosive, or primer, that explodes when the cap is struck, initiating a propellant charge or other firing train. While stopping people from shaping metal into a certain form is certainly not a practical way to worldbuild, making it so priming explosives are a chemical impossibility seems to be like a decent approach to this problem. Furthermore, blocking the percussion cap from being invented has the desired effect -- without percussion caps, you can't have weatherproof, high-reliability cartridges, nor can you have easily operated breechloading firearms (never mind not being able to develop autoloaders).
Therefore, I ask one question: what changes can I make to my world so that priming explosives are not discovered/invented while causing the least amount of collateral damage to other parts of the world? (In particular, biochemistry in the world still needs to be Earth-like, as far as is possible.)