Recently Aperture has stopped displaying previews for a number of photos (but strangely not all) when in split view. If I switch to browser view the previews are all there, and if I then return to split view they are back to being displayed. But if I move to another project and then return to the first they are gone again. I have repaired the Aperture database with no change, but not yet rebuilt it. This change in behaviour was not concurrent with an Aperture update. However there have been lots of updates affecting 10.8.2 since the most recent version of Aperture. Any thoughts?