Platies should not be choosy with their food. Have you check if the food has expired?
Sometimes you can starve the fishes for 1-2 days and try again to "train" them into accepting the food. Stop giving them tubi during this time to allow them to forget the how tasty tubi is. Haha.
I also notice most fishes displaying the "spitting the dry food out and eating them again" behaviour, this should be normal. I believed they are still taking in food just that they will not stuff themselves compared to tubi or bloodworms.
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