we removed all blocks in a 2 x 3 chunks rectange, situation under 39 height.

we did this in a place we saw a slime (hoping for one of these 6 chunks to be a slime chunk)

is there anything wrong in what you see or we just didnt have luck and none of these chunks are the one that spawnt the slime?

i believe you have to a 3 tall roof

Naw. That doesn't affect it all.
You have to note that nothing will spawn in a 16 block radius of you. So, it needs to be a long hallway.
And another note is that if you're in a Mushroom Island biome, that nothing will spawn under it until you hit the ocean-but if it isn't, then by all means, just make it longer so they can have room to spawn away from you.