I have a file geodatabase that I am working with in ArcGIS desktop 10.2.2, and when I try to add a new coded value to a domain I am confronted by the following error message:

The domain will not update. This only happens on some domains, and not others. I am not attempting to change the name or description of the domain, just adding a new coded value.
Can anyone shed any light on this? I have considered exporting the schema to an xml and trying to make edits this way, but would like to explore other options first.
Thanks in advance.