Try to check her crop early in the morning before she has eaten or drunk anything when it should feel empty. I would use a clean finger and insert it into her vent 1-2 inches to check for a stuck egg or other obstruction.
How old is she? Does she lay eggs? How does her crop feel first thing in the morning? Making a chicken vomit can be dangerous and cause them to choke to death. Can she stand and move both legs?