It's possible the rest are already pregnant or haven't responded to his display. I also think males play favorites to some extent - I have one female platy who drops like clockwork every 5-6 weeks, and another who's only dropped twice in the four months I've had her, and hasn't seemed pregnant for the last month or so.
i believe it is normal for male fish to hump ALL fish in the aquarium LOL. my friend has many fish and her males go at anything in there. But i don't think they can have babies with different species in there like a platy & guppy or guppy and neon tetra lol.