Skill tree not reflecting progress
Okay, first I thought I had not completed some of the sections, but now I see that every once in a while sections that I know I completed show up as not completed.
That is the way it works. The system makes you review weak spots and does not assume you know everything you once 'learned.'
I think what you're seeing is what Duolingo refers to as decay. There is an algorithm that calculates how quickly you can be expected to forget words without practice, and each word has a strength bar which you can see on the vocabulary tab. When the words in a lesson decay to a certain degree, the entire lesson begins to decay as well, and that's why those sections start to show up as if they had not been completed. Just do an extra practice lesson or two every day before you do a new lesson and those skills will stay golden for you.
EDIT: Doing translations in Immersion also helps to strengthen words and prevent them from decaying.