Strenghten Skill Tab
I had just started to go back to Duolingo to start studying more of Spanish and German (it had many of the vocab and the phrases we need for our Spanish Exam at school) and was shocked to find that I couldn't strenghten our skills if I was partially finished with the skill. What happened to the other part when you can strenghten your skills anytime you wanted to?
What you can do is do just practice another skill for a while and then go back to the one that you could'nt do. If you have no other skill to practice or strengthen, then you could just log out, learn the same language on a different website or app, and a few minutes later, go back to that skill. Or maybe you could just refresh the website. (I'm thinking out of the top of my head of what you can do [lol] so these things might not work.)