iPhoto 08 - thumbnails don't reflect modifications

Hi there

My wife (not a geek...) is using iPhoto for a while, she found a
strange behavior with the last version (7.1.3)
she modified a picture (Edition mode) : some effect on it  then she
exit from the Edition mode
the thumbnail doesn' t reflect the modified picture  (excepted for
cropped ones)
it's still the original thumbnail..
but when she hit the thumbnail, she get the modified picture !

I looked into the files, and there is a 'modified' folder with all the
modifications in...
maybe she is not using this new version the right way.... (with the
non-destructive functionality) ?
in this case .. any good tutorial ? (the iLife manual is tiny and not
very useful...)

thanks for helping me (and her btw...)

Erwin