Egg binding can quickly prove fatal—especially to smaller birds—so urgent treatment action must be taken. Even if you do everything correctly, there i [See the full post at: How to treat an egg-bound bird]
I have always found success by gently administering mineral oil on the vent & holding the hen in my hand with the vent over a container of boiling water such as a tea pot. The warm moisture helps a lot & by holding her in your hand you will prevent overdoing it.
The frustrating bit is when it’s a single hen in a group of the same type in the same avairy with the same diet. You then have to decide if the silly girl is not eating a varied diet even though it is being offered or if it is genetic. But when you have her sisters and they are fine you know it is less likely to be genetic.