Plurals unit has a great problem!
EVERY day I complete the lessons of plurals (all the unit, all the lessons), but in the next day it requires again review, ”strengthen”, I keep completing units, no sign of result is shown, then after reviewing units repeated times, it appears as complete again, just for start everything in the next day... That is not normal in Duo, at all, even in big units, and it has been happening for some 4 days already... I will give up...
This has been bothering me, too. I finished the tree about a week ago and try to keep it entirely gold, but some days I lose 6, 7, 8, even 10 gold skills, sometimes over the course of less than 24 hours. That just doesn't seem right. Also, a lot of skills cannot be strengthened to gold with the "practice" feature; you have to go into the lessons themselves to do it. This leads me to believe that the words and skills determining skill strength are not distributed well in the practice feature, although I'm not quite sure how all of that works.