Aperture 3.4.1 won't edit some images
I upgraded my iPhoto library to v9.4 and all seems to be ok. I then installed Aperture 3.4.1. Some of my photos I can edit ok in Aperture, but many of them cannot be edited even though taken with the same camera on the same day. Where the histogram normally comes up there is the message "iPhoto externally edited image". In the past I've done most of my editing in Photoshop Elements, with only a few edited in iPhoto. There doesn't seem to be any pattern to the Aperture problem, however. Some images that have been edited in PS can be edited in Aperture, and some that I haven't touched cannot be edited in Aperture. I'm pretty sure the same applies to iPhoto editing.
Any ideas on how to get around this problem?
Malcolm
MacBook Pro, OS X Mountain Lion (10.8.2)