I checked your website again, and this time I could replicate the issue on your website.
After debugging the issue and wasting my 3 hours I found that you have done a customization to hide the retake button if the user passed the quiz, refer:
http://prntscr.com/dncxjq
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You are not seeing the retake button because you are passing the quiz every time.
Therefore set a passing score in the quiz and then score the marks bellow the passing score then you'll see the retake button. When I shared the screenshot of the retake button I randomly marked the answers and therefore I got zero marks (failed the quiz) and hence I got the retake button.
But today while marking the answers randomly I somehow passed the quiz and I couldn't see the retake button.
Then I wasted around 3 hours to debug the issue and finally found the custmoization you made on the retake count filter which always returned zero when quiz is passed.