I'm sorry to hear that.
Having 4 more where the crop has not emptied is unusual. They hadn't eaten or drank anything before you checked this morning right?
I agree, I would switch to a pelleted feed, sometimes birds pick and choose if they have a loose grain type feed. Don't throw it out though if it's fresh and not molded, you can use it as a treat.
Can you get photos of the poop?
Do some thinking here - how long have you had the pullets, is the food fresh, could they have eaten anything moldy, anything at all changed in recent weeks? Having that many with similar issues - there's got to be a connection somewhere, if you can figure that out, it can be corrected.