Thumbnails made for some but not for others

While not an "eep, my site is giving me errors!" problem, it is effecting my layout and making my site look like I didn't know anything about layout. I want to know why this is happening, I would like a fix for it, and I would prefer not to have to install another module to handle the images.

The problem: I uploaded over 350 images and thumbnails created automatically. All functioned beautifully. The site went live, and the next three users who uploaded images, no thumbnails were created for them and the images show up full-size in the teasers. I had one of the user reload two of her images and thumbnails were created that time, but then vanished two days later. The users are following the same steps I did to upload images.

So what is going wrong? I can't find any of these images with no thumbnails in the files directory for my site. Are they stored somewhere else?

http://www.altbikermice.us/ is the URL.

Thanks for any feedback.
The BookWorm