Have the other hens been picking on her or vent pecking? She has some feather loss, and it looks like hardened crust under the vent. It would be good if you could bring her inside and soak her for 20 minutes in either warm water with Epsom salts or some Dawn or Betadine. Wash off the poop and gunk, when you get ther dry, take a new picture. While she is in there, use a glove it you have one, and insert one finger inside the vent an inch for a stuck egg, egg material, membrane, etc. Make sure that she is dried with a hair dryer and completely dry. Apply some plain Neosporin ointment to her vent area. Can you see what is going on under her belly in the first picture?