I built a mob farm without following a plan. Big dark cube with spawning area inside, only no mobs spawn inside at all. I don’t know why, as I’m not that familiar with spawning rules on bedrock. On each level inside, there is a checkerboard pattern of cobblestone and open oak trap doors. My thought would be that they would spawn on the cobblestone, try to move, and fall down the trap door to the next level. However nothing at all spawns, let alone falls down. I’ve torched off all caves in the area, still no spawns. Any tips?

Edit: They are whole blocks of cobblestone. The pattern is like this:
C=Cobblestone Block
T=Trap Door
Level 1 -
CTCTCTCTC
TCTCTCTCT
CTCTCTCTC
TCTCTCTCT (etc, 9x9)

Level 2 -
TCTCTCTCT
CTCTCTCTC
TCTCTCTCT
CTCTCTCTC (etc, 9x9)

it was supposed to work so that when they fell through a trap door on one level, they would land on cobblestone on the next level, so they would avoid fall damage. Each level is 2 blocks high so endermen won’t spawn.