If it's not ammonia and she's not ill, she could be stressed out by the bullying. I've just had to separate my male platy from his females, because he kept picking on one of them and she was just losing all confidence, staying in one corner, or going up and down the same stretch of glass. Now that I'm getting a better look at her I realise she has also lost weight (but thankfully no symptoms of illness). One effect of the stress seemed to be to make her a bit slower in her reactions, finding it a bit more difficult to identify the food for instance. Come to think of it, I've known stress to have the same effect on me (though I tend to eat more, which is possibly not such a good thing either). She has picked up amazingly since he went and is now joining in with the others.