Apologies if this is a duplicate report!
Often when attempting to upload multiple files using the Upload Wizard on Commons the thumbnails do not generate. This is very problematic for bulk uploads as the user is unsure which file is which and hence unable to provide an appropriate description or category.
The issue is not persistent. It appears if you upload one file at a time the thumbnail will always generate. Otherwise it's a hit or miss. You may need to try multiple times to reproduce the issue.
Observed on Chrome 40 and Firefox 45 on Mac OSX, and only on Commons (not aware of other wikis with the same UploadWizard).