I agree Platies are supposed to be peaceful fish. But as we all know that like people, they each have their own personalities. And we are bound to get a bad apple every now and then. I wouldn't get too discouraged and would replace the lost fish with a couple more and keep tabs on how they all get along. I have also heard as mentioned before that 2 females to 1 male or all females is recommended. That spreads the attention around a bit as those frisky boys will chase those girls. I personally have 2 females and 1 male and they get along swimmingly. Be advised mixing males and females is a good way to start a Platy family though.