Why does not my Minecraft monster trap work?

I have the Java version (so it should work)
It spawn extremely few monster inside (on normal and heavy)
it is pitch dark
2 floors high (ie 2 rooms with 4 waterways that let the mobs fall down to a chute)

Vllt picture?

I'm too lazy for a picture here to do it. I can also bad later here reinstate a picture except I go to imgur and lad something hoch.hmm. But normally the room is pitch black and 2 blocks high. Would have to go

Monsters spawn well from an area of 16 blocks away from the player.

Maybe there are many caves nearby where the monsters spawn? THEN you should then possibly illuminate. You also have to be within range for the monsters to spawn (but I think you know that.)

Make it 3 blocks high

Why should the caves affect my trap?