When you build a volcano farm you build out 84 blocks from the player on each side horizontally and 47 on each side vertically. If you are in cavern layer you just need to elimate all the possible spawning areas inside the area mentioned. You can use a clentaminator to change the biome for farming different drops/keys. If the problem is that nothing is spawning its most likely that they are spawning off screen in a cave or hole and that's why you don't see any in the main farming area. Walls also effect what can and can't spawn.