A student is completing units but they are not registering as completed or golden.
Yes, the lessons show as completed but his skills have lost the golden icon. There seems to be a glitch because when he completes a practice he doesn't even get partial credit. From what he's told me, it's happening to all the skills he's been practicing. He is the only one in the entire class without a single golden icon and he's completed 18 skills. Could this be happening because he waited too long to practice the skills? Yesterday he practiced on various skills and it only registered 1 practice on his chart.
Duolingo's algorithms don't actually directly keep track of how strong a skill is. They keep track of the strength of individual words. When enough words are weak enough, the skill shows as needing refreshing. Sometimes so many words have weakened that you have to strengthen the skill more than once to get it back to gold.
There's some more detail about how Duolingo keeps track of our learning here; http://making.duolingo.com/how-we-learn-how-you-learn
...but there's also a bug that appears from time to time. If that's what's happening to you, re-do the lessons and the skill will strengthen.