Hey Andrew,
This has been consistently happening to me. It seems like if I add too many uploads at once. It freezes midway through. If I add 2 uploads at a time, the uploads complete. I usually add 8 uploads to a review. H.264 is the codec. Some uploads are a couple gigs. They are not the smallest files. It seems like the software has a timeout feature that is stopping my uploads from completing even through there is progress on the upload.