Enemy spawning problem

I am reverse engineering the space raiders demo, and trying to implement enemy spawning using invisible sprites. The condition I'm using is EnemySpawner.Y >= ViewportTop("Game"), and it is almost working.

I just have one problem: if I have two or three EnemySpawner sprite objects along the same height, instead of all the sprite objects spawning enemies, only one object spawns and the rest disappear.

I was wondering if anyone knows how to solve this?

dropbox.com/s/tzb61wdexjlazgh/viewport2.capx