Hi Megyn. It looks like it's because you have the 3-5 Jobs window maximized. Pin will not be an option when the window is full screen as this would force the custom fields window out of view. This is explained in this Knowledgebase article.

check the security properties of that field, or if the job is set up for job level security.  Those are the only two things I can think of

Guess I should have read Greig's reply first