Inconsistent behavior of thumbnails images

I am using a call to attachmovie and loadmovie within a for loop to load some thumbnails images and display them in my app.

It works fine when I run Flash (all the thunmbnails show up). It also works fine when I load the app on my browser (Mac machine.)
On my husband’s pc some of the images are missing. On my client’s Mac, none of the thumbnail images show up.
So this is not a platform issue. It doesn’t seem to be a Flash issue.
What could it be?