Extra Skills can't strengthen
When I try to do Strengthen Skills, it takes me back to duolingo.com. However, I can do individual lessons. Is this a bug? (The extra skills are idioms and flirting)
Still unable to strengthen idioms directly, but I just did a general strengthen skills (button on the right of the tree screen), which came up with several idioms, so I have succeeded that way!
Since the topic is a few months old, I thought I'd give it a bump. I still have difficulty strengthening the extra skills (Flirting mostly, sometimes Idioms). It either takes me back to the home page or takes me through a lesson that is the same sentence 20 times and then doesn't count for XP or as being strengthened. Not really a big deal, but the completionist in me wants to keep everything topped off.
As of February 29, 2016, I cannot strengthen idioms. I get the same problem as the OP, in that I am returned to the main DL screen.
I cannot turn the Idioms skill gold no matter how many times I do the Weakest Words lesson.
Interesting! I just confirmed my email address via the in-email browser on my phone and then it took me into Duolingo where it shows that my skill is now gold. But in my Android app, it still shows as needing strengthening.