New user interface format
The new format for the Home page took me by surprise. While I don't have a choice other than to use it, I am a little confused and chagrined. It appears that Duolingo has disregarded all the completed lessons I had completed so far. What I had been doing is learning the lastest new lesson topic, then going back to an earlier lesson and doing the "strengthen" topic to review that earlier lesson. Now when I return to earlier topics, I cannot do the "strengthen" lesson, but rather must again go through the individual lessons for that topic one at a time. Or am I missing something?
Now, the skills are all set in "levels". The higher the level, up to 5, the more difficult the exercise that you will encounter. You can strengthen a skill, but first you'll be progressing the skill up to level 5. Once it's there, you'll have unlocked all the exercises that that particular skill has to offer, and you'll be able to strengthen that skill as much as you like.
The developers at Duo stated that they didn't feel comfortable placing anyone at level 4 or 5, so most of your skills might have been set to level 3. Chances are, there are new exercises that are more difficult then before in these skill trees and you'll need more practice in them now. Hence why there's no strenghten, but, rather, a practice button, instead.
I'm pulling all of this information from this thread. There's a lot more in here then I've stated, so maybe check it out. It'll probably answer some questions for you.