Depending on the selection (of a measure's child objects) a function may or may not work. And the reason for this is not obvious.
Insert One Measure works when any of the following objects selected:
• a note
• ending barline
• chord symbol
• staff text
• key signature
Insert One Measure does NOT work when the selection is:
NOTE: Insert One Measure works with many types of multi-object selections in the first list above.
The technical reason for this is, the elements on the first list belong to a single point in time guaranteed to uniquely identify a single measure. The items in the second list span time, potentially spanning measures. So it's likely to produce surprise at least some of the time. Harmless, though, to implement it anyhow. Is there a real world use case for selecting a sour prior to inserting a measure? As always, understanding how this actually affects normal usage of the program would help prioritize the request.