No longer able to strengthen two skills at once?
I remember in some of the previous practice sessions, they would have exercises that covered more than one skill that you had learned, and if the practice sessions had enough of these exercises that covered two skills, it would count as having strengthened them both. But now I've noticed that on the phone app, there's no more "practice" button, which means you have to pick the skill to strengthen, and as such, you only strengthen that one during the session. And while the browser version still has the "strengthen skills" button, lately it also seems to be strengthening one skill at a time. Did Duolingo do this deliberately, and if so, why? I think strengthening multiple skills at once not only is convenient to get you caught back up, but it also gives you practice forming a broader range of sentences, which leads to greater fluency.
You can strength skills that are at the bottom of the tree, they contain the skills of previous ones that are higher on the tree.
I've actually had skills further down the tree strengthen when specifically selecting one nearer the top. How this happens, no clue, but it's happened a lot more than once.
I strengthen specific skills and occasionally (including today) I get the targeted and another skill strengthened at the same time. It doesn't happen often but it happens.
I use the web version and I have also noticed that I only strengthen one skill at the time, I have not strengthen two skills at the same time for a very long time now. The same goes for my Android tablet app version that I use from time to time. Or the Windows phone app, for that matter.
I suppose practice makes perfect ...
"Did Duolingo do this deliberately, and if so, why?" More practice sessions=more ad impression served.
If you've got the health system, then general strengthening has been moved to the health interface.