Why aren't I able to manage custom field values for some fields?

[Peter Cornell]

I've created a new custom multi-select field (Planned Sprint) and created a context for it for the target project. There are some existing custom fields already available to the project created by someone else. The field value options for some of these existing custom fields can be updated from the project settings screen under Manage Field Values. However, my new custom field (and some of the other existing ones) don't appear in the list. 

List of fields missing Planned Sprint in Manage Field Values under Project Settings:

free_field_choice_02 context:

Planned Sprint context:

Is there a setting somewhere that I'm missing? I can't find any documentation about it. I'm able to edit the options as a system admin but ideally we'd like the project admins to be able to manage them. 

[Nic Brough -Adaptavist-]

Do you have advanced admin permissions in the project?

[Peter Cornell]

Yes, I do.