Cannot select items in Scene window

It's really nice to be able to click on items in the Scene window and select them. However, for some reason, more often than not, clicking on them in the Scene window does nothing for me. I have the "Move object" widget selected in the upper left-hand corner, and I am not in play mode. I can still select items from the Hierarchy view, but, of course, that requires me to know their names.

Does anybody have any reason why sometimes I cannot select objects in the Scene window?

Thanks!