Currently, Workfront only allows you to upload a maximum of 5 proofs for a document. If you have to make edits to the 5th proof, you need to upload a brand new document and start a new proofing string.
I didn't know this was the case. Although we don't often have more than this, it does happen and could cause issues from an audit perspective we have to have them saved in batchs of 5.....