Track Changes function thinks I'm two people

I am a copyeditor, which means I use the track changes function all the time. Since I switched to Word 2013 I've had a problem with some documents whereby after I close the document and open it again, the track changes function treats me as a new user. The problem with this is that I often go over any given bit of text several times, potentially making new changes each time, and when the system treats me as a new user, any earlier additions that I later delete show up in the margin as deleted text (rather than simply disappearing).  This is confusing for my clients, who only want to see my final changes and not the history of my train of thought. Of course, I can resolve the problem by rejecting the change instead of deleting the added text, but (a) I don't always remember to do that, and (b) this is still awkward, especially where I rewrite a given bit of text several times, making new additions and deletions each time. Can anyone advise as to how I can alert the system that when I return to a document, it should treat previous tracked changes as still active? I know there must be a way to do this because the problem only arises with some documents. I suppose there is something about the settings in the original document as it is sent to me that affects this, but for the life of me I can't figure out what that is. Thanks to anyone who can help!