Do you have it with all buildings ? I believe the feature does not work when the building is “too far” from being a rectangle.
Another possibility is that the building you click is not “an area”, but composed of multiple lines. I believe it does not work in that case.
Can you try it on buildings in a totally different area ? Or perhaps with another browser.
p.s. I hardly ever use iD, so I can only suggest from what I have read/seen elsewhere