If a console has been used by another user prior to you working on it, it is possible the headphones have been omitted from their set-up in the previous show and as such, the console has not been set up for a user with headphones.

To quickly remedy this, do the following

Open a show hit the safe scene/OK close the show 
Go to home/preferences/config
Then select both preferences and global defaults (see image below)
Now check your patching page -  the patching should have returned and your headphones should work when a solo is activated.