We have two servers load balancing with the same code on both servers. Both servers use a single upload directory and a single mySQL database for wordpress. When we upload a image it creates only the base image and doesn't create any of the smaller images and we don't receive any error messages.
We have a staging environment with a single server we can upload an image and we end up with several images of various sizes in the upload directory.
The wordpress in both our live and staging environments have the same configurations and the same versions of everything.
We have no other plugins loaded.