Big Problem With the Mobs , Also with the monsters in the whole map.

Hello, Theres a Big Problem with the spawn of the mobs and the monsters.
It is an annoying problem, The mobs like cows,sheep,chickens, spawns in low amounts, sometimes it is very difficult to find a sheep.
And the Monsters do not appear, not one, all over the map there is not one.
Only appears if i spawn a Monster with an egg.
The are no monsters in the day, and in the night, its boring,its like peaceful.
I Play the [lexicon]Minecraft[/lexicon] 1.7.2 , Bukkit 1.7.5
The difficulty : Hard
The Gamerule doMobSpawning is true
The Gamerule doMobLoot is true
Gamemode: Survival
i dont know what is the problem D:
I want to fix it, but i cant .
Please write your solutions, Thanks.
And Sorry for my Bad English

Yeah, its enabled, Monsters - enabled
Animals - enabled
i dont know what is the problem

Plugins Bukkit 1.7.5
BloodFX 1.7.2
HealthBar 1.7.2
SpawnMob 1.7.2
Chairs 1.7.2

I try to reinstall the server many times, but Not work.

And Monsters sometimes appear only in the night in small amounts Only when I'm alone, but when a second player enters the monsters disappear.
its like Peaceful

No, the plugin Spawnmob is disbaled, and i reinstall the server but dosnt work.
the monsters dont appear