View and compare proof versions not working properly for me

The files have to be in the same folder (or more precisely, you have to have somehow selected the files you want to compare. This will either happen if the proofs are all versions of the same file, or if you have two different files selected at the same time, which can't happen unless they are in the same folder)

You can only use the compare proofs feature if you're versioning the files correctly. If you have 2 disparate file uploads, even if within the same doc folder, if the latter isn't a new VERSION of the same proof, you won't get the compare tool. When uploading a new version of a proof, be sure you're going to add new > new version > proof, not just add new > proof.