There is a subtle, but very meaningful difference between availability and eligibility. The Availability Calendar is only one of over 30 different systems that are used to determine whether or not an official is eligible for a particular game. If you're unsure why an official is being deemed ineligible for a game, the best place to start would be to review the Diagnostics table in the assigning window which will describe exactly why the system has deemed an official ineligible for a game. This will point you to the exact system that is causing your ineligibility. In most cases, if you're asking this question, then it's not the availability calendar that is causing the ineligibility, but rather one of the other 29 systems is preventing the official from being scheduled. Reviewing the Diagnostics table will allow you to pinpoint the exact reason to determine how best to proceed.
Open the Assigning WindowYou'll need to open the assigning window for the specific game you're looking to assign for.
Select View Diagnostics in the Assigning Window Toolbar
Review the Diagnostics table for the "Reason" causing ineligibility
Now What?You'll need to edit or adjust the system causing the ineligibility. In some instance, you may need to use the "Show Ineligible Officials" menu option to schedule the official from there to override the system if you're sure you want to schedule them.