It's overall nutrition you need to look at.
Still not sure what this 'poultry feed' is...or why they weren't eating it.
But if only one bird is having problems, then it may not be the diet but that one birds physiology. Which actually could be traced back to poor nutrition as it grew?
What exactly is this?
All the extras are fun for the keepers, and sure the birds like to eat them...
...but might not be the best nutrition.
All they really need is a good chicken ration, and plain fresh water,
anything extra can dilute that perfected nutritive balance of the ration.
Is she a lot smaller than her hatch mates.
They are all laying well with no problems?
Maybe her pelvic points are not of a good size.
Compare her to the other new layers:
https://www.backyardchickens.com/articles/who-is-laying-and-who-is-not-butt-check.73309/
Yeah, you don't really want shell fresh from the beach, especially if it's salt water.
The calcium helps with muscle contractions as well as forming the shell.
What all and how exactly are you feeding?
Have you taken a look at her vent?
A video showing her struggles may help here.