First and foremost, make sure you see a blue message above the video that says "your assignment has been submitted. You can exit the page". You should see this message once you reach the very end of the video; wait for a few seconds to allow the grade to be sent.
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Don't leave the video quiz open for more than 6 hours - your session can expire, and even if you finish the quiz, your grade may not be recorded.

Avoid using Safari on Macintosh computers. on iOS (iPhone, iPad) devices, you need to use Safari but with the "prevent cross-site tracking" setting turned OFF. (see this page for more details: https://help.vidgrid.com/en/articles/2554558-third-party-cookies-blocked )