I had to strengthen 8 skills because I missed one day, but why? I mean I know words fade from your memory but not 8 skills in one day.
Duolingo uses a spaced repetition approach. It shows you things you've learned at times that will most help them stick in your head. If you keep the tree gold, you'll find it works really well. Sometimes that means you don't make any progress through the tree for a while - that's fine.
Duolingo takes account of how long it takes you to answer the question, and it takes account of whether or not you "peek" - whether or not you hold the mouse over a word to see what it means. When you can answer the questions quickly and without peeking, the skills will decay more slowly.
After you've strengthened the skills a few times, they'll start decaying more slowly, and then you'll be able to get further through the tree.
It appears that they keep scores in floating point, but only display them in integer, so your ''5' skill may have been close to the line. I had 7 skills to refresh today, too, and spent most of the weekend doing nothing but strengthening skills. And my fluency number went down 1%, too. I blame the latter on the removal of the flashcards from my account (part of some kind of A/B test, it seems), so I haven't been able to do flash cards since last Thursday.