PS - please don't think of it as "pushback," people here really only want to be helpful and help you do what's best for your flock. 
Edit: typo

Edit: typo
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I have 4 leghorns (have 18 hens total plus 1 young cockerel). I’ve had a lot of issues the last 6 weeks but the last 2 weeks egg laying has really Picked up. But I’m having a almost daily leghorn egg problem.
The egg is usually on or under the roost boards. It always flat/deflated, sometimes with the insides leaked out. The shell is rubbery.
I feed flock raiser and have eggshells available free choice. I rarely see any of the leghorns eat the shells but actually say one chowing down on them this afternoon. Another one took a quick nibble and moved on. Also I have a natural Sandy floor
that is littered with oyster shells.
I have tried to narrow it down to the affected hen but as the egg laying happens so early I can only go by which other leghorns lay. And they use 2 laying boxes in different areas of the run and I can’t see both boxes at the same time so I don’t know for sure who laid. Just when I think I know who has the issue she lays a good egg on the same day the broken egg also happens. But right now my best guess is it’s Angel.
I did try separating 3 of them and feeding them some eggshells mixed with scrambled eggs a few days ago. Isis gobbled hers down but Angel and Bo Peep were too freaked out by caging to eat or drink. All 4 leghorns tend to be the first to roost at night so they come into the coop first. I’ve put out moist flock raiser with a small amount of eggshell sprinkled on top for them to have a go at right before roosting. Only 1 or 2 will pick at it - I think they are too intent on roosting and not very hungry as they’ve already filled up.
I also recently put out a feeder that has layer pellets with the oyster shell incorporated into it. Any hen can eat from that. I did see one leghorn eat from it as well as a few hens but overall the flock raiser crumbles is hands down the feed of choice for all of them.
What should I do? I can house 2 most likely suspects individually in separate areas within the run and ride out their unhappiness until they calm down and hopefully lay (or don’t). I’m concerned the stress of separation will only make things worse overall. I’m also concerned that the consistent egg issue is due to a very bad underlying issue. I have health checked everyone twice within the last few days. Since I now know to look I do see small bumblefoot sores on 2 of the leghorns. This is a new issue for me to deal with and I haven’t had a chance to research how to handle it. I see no other health issues.
I am also curious about how the timing of the egg laying. It’s always there - if it happens that day (say 6 out of 7 days) - so early in the morning. If it takes 25-27 hours to make an egg wouldn’t the laying of the bad egg be getting later each day? It makes me think the egg is dropping out of the hen Rather than being properly laid and she may not know it’s happening.
All The leghorns are
Acting and pooping normally.
Any guesses as to what’s happening?
Umm... many people feed Flock Raiser(Purina) for the birds entire lives.Your hens are way too old for flock raiser