Sometimes this happens with students who submit large files over a slow internet connection. What is really happening is navigating away too soon, and so the student needs to wait on that page until it updates to show that the submission is complete.
I am a parent asking for help. My son's work showing submitted on his end but the teacher end shows it wasn't done or turned in. I have taken pictures on the zero on the assignments as well as the submitted date it shows it was done on his side. This is becoming frustrating on both ends. How can his work be graded if it isn't seen on the teacher's end? I have asked if it could be that his chrome book was issued at one elementary and he had to move and switch schools. His Chrome book shows both. I can't get an answer to that as well. PLEASE HELP! Thank you for your time and assistance.
The only thing that I can think of is that the your son is uploading the file to the assignment, it shows its there, but then not clicking the submit button. This has happened before. If you follow this guide to the end, you will see the "View Submission" section showing the submitted time and date.
If that is not the case, perhaps the student is enrolled in two courses? You mentioned he switched schools, so perhaps he is uploading to one class and the teacher is looking for it in another.
I suggest you setup a time where you can screenshare your son's computer with the teacher so you can both see what is going on and test an upload.