Unlocked guides cannot be selected

 

In an ordinary AI file, guides are light blue and, if they are unlocked, when they are selected they turn dark blue and can be manipulated. In 2 files I am working on now, suddenly the guides no longer turn dark blue when selected (even though they are unlocked). Older files of mine turn dark blue and a new file I created as a test does, but these two files have changed for some reason and guides no longer respond to selection. Any ideas why?

 

All guides in these files have been put on a separate layer from illustrations and text. Can a layer itself be locked somehow?