When you say Enhancement, do you mean a record on the "rm_enhancement" table?
If that's the case, then it is difficult to say because "Requested Item" is not an out-of-the-box column on the rm_enhancement table, meaning you are almost certainly dealing with a custom column which no-one but people in your org would have any knowledge of.
If I were you, I'd start by
1) Reviewing the dictionary entry for that "Requested Item" column. If it's not a reference field, or it's referencing anything other than sc_req_item, that would explain why it's not working.
2) Reviewing the rm_enhancment table/column ACLs to see if that could be the reason you cannot write to that field. The Access Analyzer is a great tool for this.
