Hi, i recently met a problem with GPU particles.As you can see in the following image the rest of my particles disappear when there is a static mesh between particles and my camera. It seems that the red water pipe block the rest of GPU particles.
This is proper behavior when using scene depth collision. (its a limitation)
You can try the alternative collision module, but you might want to consider faking it by using a second emitter that spawns the falling droplets.