Hi Jim,
Please indeed try suggestions from Kathy and Ben.
Overall, this is how the process works without it:
Student submits a file.
You go to the assignment and click on that file. It opens. Note that it opens on your computer, not in Moodle. Once you make changes - you make them locally. On the background this file is already downloaded on your computer to the temp files. Thus, once you click Save Changes in PPT, your computer suggest you to save it somewhere else locally, rather then in its temp folder. It cannot save it to Moodle, they are not connected
So, once you click on student file, I would reccomend to save it somewhere locally right away, thus you would not lose your marking changes.
Then correct the work. Login back to Moodle, go to the assignment submission and provide corrected new file back (via the Response file in the Advanced uploading of Files assignment)
hope this helps