platies that never get pregnant?

[agaul23]

I've had these platies for nearly 4 months and not one has gotten pregnant or had fry. Are they sterile?

 

[Gillie]

That or they are all the same sex

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[agaul23]

There's definitely 4 females in the bunch. That's disappointing to hear, n sad for the fishes

 

[agaul23]

Very surprised today, I noticed one of the females is pregnant, she has a swollen belly, very happy to c that. Just slow breeders I guess