Used to work fine. Am I missing something obvious here?!
OS X Lion 10.7.5
That what I suspected. The loupe is one of the features that does not work well with Aperture 2 in Lion and Mt. Lion, that is a known bug.
For full compatibility with Lion and Mt. Lion you will need to update to Aperture 3. The version that is now available from the AppStore, is no longer as expensive as the earlier versions of Aperture used to be.