These are probably stupid.......

I built a mob grinder, expecting to collect armor and iron from zombies, but still nothing. Is that not an option on this server?

I also have a "field" off in the distance where I hoped to spawn cows, sheep, pigs, etc., but again, nothing. With mob spawning enabled. Did I miss something?

 

I have been experiencing the same issue. I built a spawner and very minimal amounts of mobs actually spawn. I don't know the reason, but I will tell you some peoples spawners are extremely efficient while others with the exact design don't even work.

 

The hostile mob spawned works fine (except I never see armor or iron drops, but that might be expected). But no passive mobs spawn on my grass. I did, however, see a chicken in my mob grinder....... ?

 

I don't think passive mobs will spawn naturally, you need a spawner for those (correct me if I am wrong).