http://www.the-chicken-chick.com/2012/04/prolapse-vent-causes-treatment-graphic.html check this out for more answers hope it helps you I also looked for same thing and this is what I found...also found this if this helps any...https://www.backyardchickens.com/t/637621/my-chicken-has-a-prolapsed-vent
have you tried giving them (grit) it also helps them to produce better eggs and with the Oster shells this is what I have given mine and now they are doing great.
one of my hens was also doing this soft egg shell so I gave her some grit and a higher protein base food and it sure did help. this also helps for molting warm water and dog food for a treat.
WOW love it ...great story...just getting started this year and having soooo much fun so far...hope to move some day on a little farm or bigger property and be able to have a little bigger chicken coop and a few more chickens...just love your coop and the girls for sure will love their new home...