Hi...
I checked your website and found out that the issue is with the iframe code you are using in the unit.
This code is removing the section which shows the previous/next unit buttons, refer:
http://prntscr.com/fa0fr2
This section is removed and therefore the unit is not marked as complete.
You can remove the iframe code and then check it will work fine.
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You are using the iframe for survey, you can also create a separate survey type quiz in wplms also, simply create a quiz which is manually evaluated and then add the survey type questions in it.