Hi there .. we ran into the same issue, specifically with TIFF files. Ours were generated directly from the microscope, so we thought maybe it had to do with the layers?? The files were pretty large, though. We also realized that we had not set a max thumbnail size limit, so it was still trying to make a thumbnail, even for the larger files, and would just hang forever and never upload the file. Our depositor finally decided to upload the original file w/o a thumbnail (ie: if you limit the size, if it goes beyond that it just doesn't generate one), and then make a smaller version of the file, upload it, and get a thumbnail on that one. I believe there is a way to generate your own thumbnails and manually upload/copy them to the directory where the main files are. If you give them extensions of .thumb64 and .thumb400 (or maybe .thumb48) and size respectively .. one of those should appear on the thumbnail box of the file.