Did you create the field with the Jobs app? If that's the case, the fields should be available for new and old jobs of that domain.

Creating a new field in the app can be done like this:

This field will then be available on jobs for that job domain:

There are some scenario's where the field might not be visible in the UI though.

This will be the case when a field or section is marked as hidden. This will be the case when a field/section was deleted while there were still jobs that used the field/section:

Anothing thing to consider is that the fields will not be displayed in the list when the setting "Show in list view" on the field is disabled.

Something else to be aware of is that the section definitions are being cached in the app. So having the jobs app open (either embedded in a Cube or in your browser) will not show new or updated fields that were created in the background. A page refresh is required for those changes to be taken into account.