Up until recently, SourceTree has been showing preview images for SketchUp files.

But for one SketchUp file, it doesn't show preview images.

Anyone know why this might happen, and better yet, how to fix it? I'm wondering if it has something to do with how I save the file, but I can't figure it out.
I really like the preview images, because I can see the changes I've made, at a glance.
Thanks in advance.