In one of my Terraria worlds I've created a Volcano farm that looks like this:
It is located at
X 0 Y 862 (Underground), and I've cleared all the surroundings:
Note that I left a Stone platform for mobs to spawn in. I was expecting mobs spawning and moving towards the player to die in the lava pool, but even if that is happening, it happens at a very low rate (about 4 mobs per hour). I know that the spawn rate should be higher so, what am I missing?