I have a picklist field on a custom object that is disabled (grayed out) when I am editing a record on that custom object for a user in a profile that should have write access to that field.
I have verified the user's profile should have the ability to edit the field under "Field Accessibility" for that field (field-level security). I also have verified that the field is not "Read Only" on all of the object's page layouts.
This field is editable using the System Administrator profile.
Where/what else might I check?