Mob Spawner Spwans Mobs in wrong place

There is two Shulker Spawner in the end.
When I stay with the Spawner in end city, I cannot see any Shulker, but the Spawner is keep spawning.
When I back to the main island, Shulkers all there! And there are keep spawns new Shukers!
After I destoryed the Spawner, there is no Shuker now.