Java provided attribute doesn't populate value set on modifying of work item

I've created a Java provided attribute for a work item that provides a value set by looking at two other attributes (enumerations) from the same/current work item which together form a directory path that the java provided attribute will create a value set using the names of files nested in that directory. It works well if a work item is created for the first time however on modifying the work item, it fails to populate the set so a new value can be selected.

I'm not sure how to have the java provided re-populate the set so a new value can be selected. I have de-bugged it and seen that when it tries to get the values of the two enumeration attributes that it returns an empty string each time though the attributes have values set.

Below is a link to my class file.

http://www.megafileupload.com/hPqE/SpecificArtifactType.java

Any help appreciated.

Thanks