Hostile Mob Spawning Chance?

I built a mob spawning tower and not many mob spawning. Is server hostile mob spawning chance problem or my mob spawning tower problem ?
p.s. I am not that good at English, sorry if my grammar is bad.

 

Is it sufficiently dark enough for the hostile mobs to spawn? If not, then I think that's your problem.

 

Yes, it is dark enough for mob spawning